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Chapter 8 Quantitative Analysis of Secondary Hydrocarbon Migration Systems
227
CHAPTER a QUANTITATIVE ANALYSIS OF SECONDARY HYDROCARBON MIGRATION SYSTEMS
A quantitative analysis of secondary hydrocarbon migration systems should result in figures for the volumes and compositions of hydrocarbons migrating in a sedimentary basin as a function of time and space. Ideally, all aspects of a secondary hydrocarbon migration system in a sedimentary basin at a certain time during the basin’s evolution, should be quantified in the analysis, i.e. the masses and initial composition of hydrocarbons available for secondary migration, the three-dimensional migration pattern, the flux of migrating hydrocarbons and the migration losses. Different procedures are available for the quantitative determination of the masses and initial compositions of hydrocarbons available for secondary hydrocarbon migration (e.g. Duppenbecker et al., 1991;Mackenzie and Quigley, 1988;Tissot and Welte, 1984). The reader is referred to the published literature for a detailed outline of these procedures. This chapter presents approaches for the quantitative determination of the remaining characteristics of present and past secondary hydrocarbon migration systems. Emphasis is placed on the identification and integrated analysis of a wide variety of observable present-day physico-chemical characteristics of fluids and rocks in a sedimentary basin in order to quantify present-day migration characteristics and t o place constraints on the reconstruction of the history of hydrocarbon migration systems in the basin.
A quantitative analysis of present-day secondary hydrocarbon migration for basin evaluation can be restricted to the prospective parts of a sedimentary basin as selected on the basis of the previously described qualitative study (Chapter 7). The quantitative assessment of present-day hydrocarbon migration systems is described separately for hydrostatic and hydrodynamic conditions of the prospective parts of the basin (Sections 8.1,8.2and 8.3). Section 8.4 briefly describes the available approaches for a quantitative analysis of the evolution of secondary hydrocarbon migration systems.
Chapter 8
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Structurecontours top carrier rock Hydrocarbon expelling source rocks Catchmentarea
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Hydrocarbon expelling source rock Drainagevolume
Figure 8.1 Map view and cross-section of hypothetical prospective area.
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229
8.1 Present-day hydrostatic hydrocarbon migration systems In order t o quantify the separate phase hydrocarbon migration under true or assumed hydrostatic conditions, the location of hydrocarbon expelling source rocks, the amount and characteristics of expelled hydrocarbons and the basin's hydrogeological framework should be known. The hydrostatic separate phase hydrocarbon migration starts in the porous and permeable hydrogeological units (i.e. carrier-reservoir rocks) adjacent to the expelling source rocks. As outlined in Chapter 4, secondary hydrocarbon migration in hydrostatic basins is a preponderantly lateral migration through carrier-reservoir rocks. After expulsion from the source rock, hydrocarbons will move updip along the upper boundary of the adjacent carrier-reservoir rocks until traps are encountered. The migrating hydrocarbons will seek the shortest possible migration paths. In order to establish the hydrocarbon migration pattern, the geometry of the upper boundary of these carrier reservoir rocks and the location of permeable and impermeable zones along this boundary should be derived from the known hydrogeological framework of the basin (Section 6.3.4). The hydrocarbon migration pattern from source rock to potential trapping positions can be constructed with the help of a depth-contour map of the geometry of the upper part of the appropriate carrier-reservoir rocks (Section 7.1.2). The geometry of the upper boundary of the carrier-reservoir rock and the location of vertical seals along the migration path determine the possible trapping locations. After having identified the potential hydrocarbon migration paths from expelling source rock to a trapping position, the total volume of hydrocarbons lost WL) along the migration pathways can be estimated from the total volume of rock through which the hydrocarbons migrate (the drainage volume VD) and the mean porosity of that rock (n) (Section 4.3.2, Equation 4.25: V, = n&VD; S, = apparent residual saturation, estimated at 1 - 3%) as proposed by Mackenzie and Quigley (1988; Figure 8.1). Knowing the volume of petroleum expelled from that part of the source rock that provides a drainage area for the trap being evaluated, the total hydrocarbon charge that potentially is available for the trapping location can then be estimated from the difference between the volume of hydrocarbons expelled from the source rock and the volume of hydrocarbons lost during secondary migration (Mackenzie and Quigley, 1988). Repeating this procedure for all potential trapping positions along the identified migration pathways in the studied part of the basin, the potential trapping locations can be ranked according to hydrocarbon charge. The volume of hydrocarbons that actually have reached a certain trapping position in a presently stable hydrostatic basin is, in theory, also influenced by the time required for the hydrocarbons to reach the trap in relation t o the time
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230
elapsed since hydrocarbon expulsion from the source rock started to the present-day. The updip lateral migration of oily hydrocarbons proceeds at a specific discharge rate in the order of millimetres per year (Section 4.1) corresponding to velocities of tens of centimetres per year. Hence, the specific discharge rate will probably not be a limiting factor on the hydrocarbon charge for the generally encountered migration distances of less than 30 km (Section 4.3). Quantitative information on migrating hydrocarbons can be obtained by using the specific discharge equations given in Section 4.1. For example, the specific discharge for hydrostatic updip migration can be calculated from the equations
-2
r
khc = T n S h c
8Z
To solve these equations, requires knowledge on geometrical and hydraulic properties of the rocks (Section 6.3.4), densities of groundwater, and densities and viscosities of hydrocarbons. The density of the groundwater can be estimated from Figure 1.3 for different temperatures and salinities. As a first approach, the densities and viscosities of hydrocarbons estimated from, respectively, Figure 4.7 and Table 4.2 can be used in the calculations. The reader is referred to England et al. (1987) and Mackenzie and Quigley (1988) for more precise techniques to determine the density and viscosity of hydrocarbons for various subsurface conditions and for different compositions of the hydrocarbons. Different modelling approaches simulating hydrostatic hydrocarbon migration have been developed (Lehner et al., 1987; Sylta, 1987, 1991a, 1991b). Figure 8.2 shows the result of an example calculation, as given by Lehner et al. (19871, for hydrostatic separate phase hydrocarbon migration through a hypothetical carrier rock of indefinite thickness at different times during subsidence. It has been assumed in the calculation that the location of hydrocarbon input into the secondary migration system shifts with continued subsidence. Sylta (1991a) incorporated modelling of phase behaviour in the modelling of secondary hydrocarbon migration by using an equation of state of a multicomponent hydrocarbon mixture. Sylta’s modelling approaches also account for migration losses (Sylta, 1987, 1991).
Quantitative analysis of secondary hydrocarbon migration systems
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Chapter 8
8.2 Present-day hydrodynamic conditions
The quantitative analysis of hydrodynamic conditions in a prospective part of a basin includes the identification of the different types of groundwater flow system (gravity-induced, burial-induced, tectonically-induced), the quantitative assessment of the characteristics of the identified groundwater flow systems, the interaction and genetic explanation of the flow systems. For this purpose, a quantitative integrated analysis of direct and indirect indicators of regional groundwater flow is used in combination with groundwater flow modelling techniques. Section 6.3 provides information on the data sources for the principal direct and indirect indicators of flow (groundwater pressure; and temperature, salinity and chemical composition of groundwater). Irrespective of the type of sedimentary basin under consideration, i.e. the type of groundwater flow system under consideration, the current direction and intensity of groundwater flow at each point in a basin under isothermal and isochemical conditions are directly related to the groundwater potential gradient, the density and viscosity of groundwater and the permeability of the subsurface (Chapter 1). On a regional scale, the combination of groundwater pressure, density and viscosity data and permeability data directly indicate e.g. the lateral groundwater flow pattern, areas of concentrated horizontal or vertical flow and the location of geopressured zones. Some characteristics of indirect indicators of flow may be associated with a particular flow condition independent of the type of flow system under consideration (e.g. positive anomalies of groundwater temperatures and salinities at shallow depths may indicate focussed vertical upward flow of groundwater). A correct genetic interpretation of the direct and indirect indicators of groundwater flow and the associated groundwater flow directions and velocities requires knowledge on the type and evolution of the groundwater flow systems involved. The previously described qualitative study is a first approach to differentiate between the different groundwater flow systems (Chapter 7). 8.2.1 Hydrodynamic conditions in stable subaerial regions The hydrodynamic condition in a stable subaerial region is given by the characteristics of the prevailing gravity-induced groundwater flow systems.
The present-day groundwater flow systems in the selected study area may not be in accordance with the relief of the present-day water table, and as a consequence the characteristics of the flow system cannot be reliably inferred solely from water table relief and subsurface permeability distribution. A direct determination of the quantitative characteristics of the present-day gravityinduced groundwater flow system, requires data on pressure, density and viscosity of groundwater and data on the permeability distribution
Quantitative analysis of secondary hydrocarbon migration systems
233
supplemented with data on the relief of the present-day water table, o r eventually on the topographic relief of the ground surface. The interpretation of these data should be integrated with a n evaluation of the different characteristics associated with gravity-induced groundwater flow, as apparent at the ground surface (moisture conditions) or in the subsurface (temperature, salinity, hydrochemical composition, isotopic composition of groundwater) (Chapter 2). T6th (1978) proposed an analysing technique for the quantitative assessment of cross-formational gravity-induced groundwater flow systems and the genetic explanation of the identified systems, based on the integrated interpretation of the following five pressure-related parameters: - potentiometric surface - pressure-depth relation - dynamic pressure increment - hypsographic distribution - water table elevation.
Potentiometric surface The groundwater potentials of a certain hydrogeological unit can be calculated from groundwater pressure and density data (Chapter 1). Groundwater potentials are often expressed in equivalent fresh-water heads. Equipotential lines, or potential contours, connect points of equal groundwater potential for a single hydrogeological unit. The potential contours for a hydrogeological unit add up to a relief map of the potentiometric surface for that unit. A potentiometric surface map of a hydrogeological unit provides a regional picture of the magnitudes and directions of the groundwater potential gradients, which is also of direct importance in analysing hydrocarbon migration systems. For analysing groundwater flow conditions, the direction and spacing of the potential contours of the potentiometric surface of a hydrogeological unit can be used to identify the lateral component of the groundwater flow pattern, the location of lateral barriers to flow andor the location of vertical escape ways from the unit. Pressure -depth relation A regional picture of groundwater pressure-depth relations is indicative of the following groundwater flow characteristics: groundwater flow condition (hydrostatic or hydrodynamic), groundwater flow direction (descending, horizontal or ascending), absence or presence of pressure barriers. Dynamic pressure increment The dynamic pressure increment, Apw, at a certain depth is the difference between the hydrostatic and the hydrodynamic pressure-depth relation at that depth (T6th, 1978). T6th (1978) showed that the dynamic pressure increment is a function of both ground surface elevation and depth of measurement. He proposed the use of a two-dimensional presentation of this relation (the elevation-depth pattern: Apw-z-d)as an analysing tool. The elevation-depth
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234
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Quantitative analysis of secondary hydrocarbon migration systems
235
LEGEND c'
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pattern i s a family of is0 Apw curves plotted against groundwater head elevation, z, and depth of measurement, d (Figures 8 . 3 ~and 8.44. Such an elevation-depth pattern of the dynamic pressure increment can be used to identify the type of gravity-induced flow systems (local, intermediate, regional
Chapter 8
236
flow systems), the lateral and vertical extent of each flow system, the hydrogeological framework, the hydraulic continuity between separate porous and permeable hydrogeological units, and the effect of ground surface topography on the groundwater pressure distribution.
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Figure 8.5 Relation between hypsographic curve of the ground surface and t h a t of the groundwater potentiometric surface of regionally confined a n d unconfined aquifers (after T6th, 1978, Water Resources Research, Vol. 14,no. 5, Fig. 7, p. 813, Copyright (0)1978 by the American Geophysical Union).
Quantitative analysis of secondary hydrocarbon migration systems
237
Hypsographic distribution A hypsographic distribution is the relative areal frequency in a certain region of elevations above datum of a laterally extensive bounded surface. It can be expressed by the hypsographic curve, which is a plot of elevations representing specified vertical intervals, e.g. of the ground surface against the cumulative sums of the relative extent of the corresponding horizontal projections. The analysing technique proposed by T6th is based on a comparison of the hypsographic curves of the ground surface and a potentiometric surface of a particular region. The analysing technique may be used t o determine whether a particular part of the groundwater flow system is in accordance with present ground surface topography. Figure 8.5 gives an example of a theoretical hypsographic distribution. According to T6th (1978) this technique can be very useful in large areas with varied topography and sparse data control. Water table elevation The elevation of the water table is the level where the groundwater pressure is atmospheric, i.e. the groundwater in an observation well would rise t o this level. The elevation of the water table is the vertical distance between a datum plane and this level, which corresponds to the water table within the aquifer (phreatic surface) or to the potentiometric surface. The elevation of the water table may be derived from groundwater pressure-depth plots (T6th, 1978). This parameter may be useful for determining outcrop regions of hydrogeological units and for identifying whether the groundwater flow is regionally unconfined or not, by correlating the elevation of the water table with the elevation of the ground surface.
The interpretation of the five pressure-related parameters can be supported by an evaluation of physico-chemical phenomena associated with gravityinduced groundwater flow and a comparison of present-day groundwater flow characteristics with theoretical flow characteristics for different hypothetical conditions and/or paleogeological and paleohydrogeological conditions. The correctness of the identified characteristics of the groundwater flow systems can be verified by analysing the observed physico-chemical characteristics of the studied region in the same way as outlined in Section 7.1.3.1. In addition, the regional distribution of indirect indicators of flow, especially temperatures, may be used t o identify geohydrological characteristics of the basin, such as directions and fluxes of groundwater flow and permeability distributions. For example, Chapman et al. (1991) used temperature distributions to constrain their numerical simulations of coupled groundwater flowheat flow for the Uinta Basin, USA. By varying geohydrological and thermal parameters they could estimate basin-scale permeabilities and groundwater flow conditions (see also Willet and Chapman 1987,1989;Smith et al., 1989).
238
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For the genetic interpretation of the five pressure-related parameters the observed distribution of parameter values may be compared with the theoretical distribution of parameter values as calculated for hypothetical models. T6th (1978, 1979, 1980) applied this approach in his regional groundwater flow study of the Red Earth Region in Alberta, Canada. Figures 8.3, 8.4 and 8.5 present examples of calculated parameter distributions for two different theoretical situations as given by T6th (1978,1979, 1980). Figures 8.3 and 8.4 show the distribution of the following parameters in a hypothetical homogeneous and hydraulically continuous basin with simple and complex topographic relief, respectively: groundwater potential and groundwater flow ( Figures 8.3a and 8.4a); groundwater pressure (Figures 8.3b and 8.4b) and dynamic pressure increment (Figures 8 . 3 ~ and 8.4~).An example of a theoretical hypsographic distribution is given in Figure 8.5. The quantitative characteristics of the gravity-induced groundwater flow systems identified by the interpretation of the five pressure-related parameters can be explained genetically by applying an appropriate simulation model of gravity-induced groundwater flow (e.g. Bethke, 1986a, 1989; Garven, 1989; Garven and Freeze, 1984; Zijl, 1984, 1988, 1989). Assuming steady-state conditions in the area of study, simulation of gravity-induced flow systems resulting from the present-day water table potential distribution and subsurface permeability distribution, gives information on the geometry of the flow systems, the groundwater flow directions and velocities and the groundwater potential distributions. By comparing the identified characteristics of the groundwater flow systems with those calculated for the present-day relief of the water table and the present-day permeability distribution, the characteristics of the present-day flow systems that are in accordance with the present-day relief of the water table can be identified. The maximum depth of a steady-state groundwater flow condition can thus be estimated as well. The present-day groundwater potential distributions and groundwater flow directions in the deeper parts of the basin may still be related t o former configurations of the water table (i.e. former topographic, climatic and/or sea level conditions; Section 2.3). The interpretation of such transient hydrodynamic conditions requires data on the paleotopography of the groundsurface, or on paleoclimatic and -sea level conditions, and data on the paleohydrogeological framework. Past water table configurations may be inferred from knowledge on the recent geological evolution of the basin. The reconstructed past water table configurations and the known or assumed paleohydrogeological framework of the basin may be used as input data for a steady-state groundwater flow simulator t o determine the characteristics of past groundwater flow systems in accordance with the corresponding past water table configurations. A combination of such steady-state simulations of past gravity-induced groundwater flow conditions with calculations of adjustment times of groundwater potential t o changing boundary conditions can be used to interpret the present-day transient groundwater flow conditions
Quantitative analysis of secondary hydrocarbon migration systems
239
(e.g. Garven 1989; T6th and Corbet, 1987). Simulation of the hydrogeohistory by applying models of unsteady-state gravity-induced groundwater flow are also used to interpret present-day transient groundwater flow condtions (e.g. Senger et al.,1987). 8.2.2 Hydrodynamic conditions in subsiding and filling basins
The present-day burial-induced hydrodynamic conditions in the selected area can be assessed directly from the groundwater pressure, density and viscosity data and the permeability data. Each subsystem of burial-induced groundwater flow is characterized by specific vertical and horizontal changes of the groundwater potential (Section 2.1.3): Characteristics of the shallow subsystem: the groundwater potential increases slightly with depth, reflecting cross-formational vertical upward flow of groundwater; there is no lateral change in groundwater potential; the groundwater pressure-depth gradient is near hydrostatic t o slightly superhydrostatic. Characteristics of the intermediate subsystem: the groundwater potential in fine-grained rocks is higher than that in adjacent relatively coarse-grained rocks; the groundwater potential in the relatively coarse-grained rocks changes laterally, indicating the lateral flow direction of groundwater through these rocks; the groundwater pressures in the relatively coarse-grained rocks are superhydrostatics; the groundwater pressure-depth profile in the relatively coarse-grained rocks runs parallel to the hydrostatic gradient. Characteristics of the deep subsystem: the fine-grained as well as the coarsegrained rocks are geopressured; large vertical changes in groundwater potential occur over the fine-grained rocks and large lateral changes in groundwater potential occur in the relatively coarse-grained rocks over relatively short distances, reflecting the restricted groundwater flow conditions in the deep subsystem. A regional picture of changes in groundwater pressure with depth and groundwater potential with depth in combination with potentiometric surfaces constructed for different hydrogeological units permit the delineation of the vertical and lateral extent of the shallow, intermediate and deep subsystems of burial-induced flow in the studied area.
Subsequently, additional characteristics of the burial-induced hydrodynamic conditions of importance for hydrocarbon migration, such as directions and rates of groundwater flow, zones of concentrated flow, should be determined for each of the subsystems. The shallow subsystem of burial-induced flow can be considered to be hydraulically continuous. Therefore, the concept of dynamic pressure increment, as introduced by Tdth (1978; Section 8.2.1) can be used to
240
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assess the vertical direction and rate of groundwater flow in the shallow subsystem. The pattern of lateral groundwater flow through the relatively permeable hydrogeological units in the intermediate subsystem can be derived from the potentiometric surface maps of the groundwater constructed for each of these units. It is also important to look for discontinuities in the distribution of the groundwater potentials (such as large differences of potential over short lateral distances and, conversely, zones of very small lateral differences of potential) in relation t o tectonic and structural elements or stratigraphic features, because they may indicate the location of lateral barriers t o flow andlor the location of vertical escape ways for groundwater from the hydrogeological unit. The rates of lateral groundwater flow through the relatively permeable hydrogeological units can be calculated from the lateral groundwater potential gradients, the groundwater density and viscosity and the permeabilities of the hydrogeological unit. The magnitudes and directions of the changes in groundwater potential with depth in the poorly permeable units in the intermediate subsystem affect the sealing capacity of these units for hydrocarbons. Ideally, a detailed regional picture of groundwater potential profiles over these units should be constructed from pressure data o r pressure indicators (Section 6.3). The potentiometric surface map of the groundwater in each of the relatively coarse-grained units in the deep geopressured subsystem in combination with lithostratigraphic and structural information of these units will indicate the lateral direction of groundwater flow, location of vertical barriers t o flow, and the location of possible vertical escape ways for groundwater. The regional distribution of the magnitudes of groundwater pressures and potentials in the geopressured zone will help t o delineate possible zones of seal failure resulting from hydraulic fracturing. Zones of concentrated upward flow of groundwater, which may occur along the edges of an intermediate subsystem, along permeable faults and diapirs, and through hydrofractured zones of seal failure in the deep geopressured subsystem, induce positive pressure, temperature and salinity anomalies and associated mineral assemblages in the shallow parts of the basin. The location of the zones of concentrated upward flow of groundwater inferred from the interpretation of groundwater potential and pressure data may thus be confirmed by an evaluation of available thermal and geochemical information (see Chapter 7). The identified pressure distribution and the other physico-chemical characteristics associated with burial-induced groundwater flow are the combined result of the time-dependent processes of groundwater pressure generation and dissipation. The groundwater pressures in a filling sedimentary basin are generated by the combined effect of the increase in load of the water-saturated sediments and the aquathermal effects, which, a t greater depths, may be enhanced by the dehydration of clay minerals and by hydrocarbon generation from organic matter (Section 2.1.3). Ideally, a
Quantitative analysis of secondary hydrocarbon migration systems
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quantitative genetic interpretation of the identified present-day hydrodynamic conditions should take into account the combined influence of the different pressure-generating mechanisms and the hydrogeological framework of the basin. Different theoretical approaches for establishing groundwater pressure distributions in a subsiding and filling basin have been presented (Bethke, 1985, 1986b;Bethke and Corbet, 1988;Bethke et al., 1988;Doligez et al., 1986; Forbes et al., 1992; Magara, 1978, 1986, 1987; Mann and Mackenzie, 1990; Ungerer et al., 1987b). These approaches include two-dimensional basin-scale modelling techniques to simulate groundwater pressure generation and groundwater flow (e.g. Bethke’s model, Themis model). Bethke’s model (1985, 1986) considers the development of groundwater pressures and groundwater flow in a subsiding and filling basin as resulting from compaction, aquathermal pressuring and mineral dehydration reactions (Bethke, 1985,1986; Bethke et al., 1988; Harrison and Summa, 1991). Using Bethke’s model, Bethke et al. (1988) and Harrison and Summa (1991) simulated and interpreted the development of present-day groundwater pressures and groundwater flow in the Gulf of Mexico Basin, USA (Figure 2.30). The Themis (Temispack) model is a twodimensional model, which can simulate numerically compaction and burial processes, hydraulic fracturing, groundwater flow, conductive and convective heat transfer and hydrocarbon generation by using Darcy’s law for groundwater flow, porosity versus effective stress relationships for compaction, parallel first order kinetic reactions and Arrhenius laws for hydrocarbon generation (Doligez et al., 1986; Forbes et al., 1992; Ungerer et al., 1987b, 1990). The Themis model has been used to study hydrodynamic conditions in, for example, the North Sea Basin (Figure 8.6; Bunus et al., 1991; Doligez et al., 1987; Ungerer et al., 1987a1, the Mahakam Delta, Indonesia (Burrus et al., 1991; Schneider et al., 1991), the Eastern Canadian margin (Forbes et al., 1992) and the Paris Basin (Burrus et al., 1991). The application of the above-mentioned modelling techniques allow the assessment of the relative importance of the possible pressure generating mechanisms and the different characteristics of the hydrogeological framework in relation to the observed present-day hydrodynamic conditions in the basin. 8.2.3 Hydrodynamic conditions resulting from interactions of
different groundwater flow systems Different groundwater flow systems may co-exist and interact in a basin (Section 2.5).
When a subsiding basin is surrounded by continental areas or when part of the basin has emerged and stabilized above sea level, the hydrodynamic characteristics of such basins reflect the interaction of burial- and gravity-
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242
a
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-
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W a t e r velocity ( I 0 to GOOm/MA.)
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Figure 8.6 Temispack reconstruction of the present-day distribution of overpressures induced by compaction disequilibrium for two assumed conditions of fault permeability: a. faults are assumed to be permeable; b. faults are assumed t o be impermeable (arrows: Darcy velocity) (from Burrus e t al., 1991, Geological Society Special Publication no. 59, Fig. 7, p. 97. Reprinted by permission).
Quantitative analysis of secondary hydrocarbon migration systems
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induced flow systems. Features, such as positive pressure, temperature and salinity anomalies are characteristic of zones of concentrated upward flow along the edges of subsiding basins as well as of upward flow in discharge zones of gravity-induced flow systems. Along the edges of subsiding basins surrounded by continental areas, these features may be genetically related to either one of these flow systems or to both. The genetic interpretation of the observed direct and indirect indicators of groundwater flow in the basin should include an evaluation of both flow systems and their interactions. Bethke et al. (1988) and Harrison and Summa (1991) used two-dimensional groundwater flow modelling techniques taking into account the two driving forces for groundwater flow, i.e. sedimentation in the subsiding part of a basin and relief of the groundwater table in its continental part, to reproduce present-day hydrodynamic conditions in the Gulf of Mexico Basin. Burrus et al. (1991) showed numerical results of a version of the Themis model that includes the two driving forces for groundwater flow for the Paris Basin and the Mahakam Delta. Hydrodynamic conditions in a basin may in part result from tectonic forces. Ge and Garven (1989) applied a numerical model of coupled tectonic- and gravity-induced flow to evaluate the relative importance of tectonic influence on groundwater 'pressure and flow in an otherwise gravity-induced flow system in a hypothetical foreland basin. Forbes et al. (1992) included an evaluation of lateral compression in their numerical reconstruction of the present-day pressure distribution in the Venture Field, Eastern Canada. Free thermal and thermohaline convection may occur locally in sedimentary basins. The possible occurrence of thermal convection of groundwater should be evaluated in sedimentary basins with high heat flows and around magmatic intrusions and salt diapirs. The possible existence of thermohaline convections should be evaluated near evaporite occurrences (Section 2.4).
8.3 Present-day hydrodynamic hydrocarbon migration systems The identified types of groundwater flow system, their groundwater-depth gradients and potentiometric surfaces in combination with the hydrogeologic _",-&meworkallow an evaluation of the sealing capacity of the barrier rocks under hydrodynamic conditions (e.g. lateral continuity of barrier rocks of sufficient sealing capacity; zones of seal failure by hydraulic fracturing of rocks) and the role of faults as vertical pathways or barriers for hydrocarbon migration (Sections 5.2 and 5.3).
Chapter 8
244
’,.
I
/
.I
-
Figure 8.7 Cross-section showing graphical relation between equipotential surfaces, u = constant, and those of v = constant and z = constant for equal intervals of Au = Av = Az (after Hubbert, 1967).
The migration pattern for hydrocarbons in aqueous solution and very fine suspension can be derived directly from the previously identified groundwater flow patterns.
For parts of the basin with laterally continuous barrier rocks of sufficient sealing capacity, the pattern of separate phase hydrocarbon migration through carrier-reservoir rocks and the location of trapping positions can be determined by applying Hubbert’s mapping technique (Dahlberg, 1982;Hubbert, 1953,1967) assuming that the locations of hydrocarbon expelling source rocks are known. The application of Hubbert’s W Z mapping procedure requires information on the geometry of the upper boundary of the carrier-reservoir rocks, the groundwater potential distribution in the carrier-reservoir rocks, the groundwater density and the hydrocarbon density. Section 5.2 showed that the potential of a unit mass of separate phase hydrocarbons in a water-saturated rock under hydrodynamic conditions is given by Equation 5.2. Omitting the influence of capillary pressures on the hydrocarbon potential reduces Equation 5.2 to (8.i) Equation 8.1 is the basis of Hubbert’s mapping procedure. For constant groundwater and hydrocarbon densities, Equation 8.1 can also be expressed as
Quantitative analysis of secondary hydrocarbon migration systems
2A5
(8.2) By letting
it follows that Uhc = vhc - z. At every point in a carrier-reservoir rock, Uhe, which is proportional to the hydrocarbon potential, can be determined from the elevation z and the value of Vhc, which can be calculated from the groundwater potential (Figure 8.7). The UVZ mapping procedure results i n maps o r cross-sections showing hydrocarbon equipotential surfaces in carrier-reservoir rocks from which hydrocarbon migration directions and potential trapping positions can be derived (Figures 8.8 and 8.9). The volume of hydrocarbons lost along a migration path from expelling source rock t o potential trapping position, can be estimated by applying Mackenzie and Quigley's procedure (Section 8.1). Under hydrodynamic conditions, additional losses can be expected to occur by removal of light hydrocarbons in aqueous solution. After having estimated the hydrocarbon charges available for the different trapping locations, the traps can be ranked according to hydrocarbon charge. The calculation of specific discharge rates for separate phase hydrocarbon migration requires knowledge on the geometrical and hydraulic properties of the carrier-reservoir rocks (Section 6.3.41,the densities of groundwater, the densities and viscosities of hydrocarbons (Section 8.1) and the groundwater potential gradients in the carrier-reservoir rock. Specific discharge rates for separate phase hydrocarbon migration under hydrodynamic conditions can be calculated from the following equations given in Sections 4.1 and 4.2
An evaluation of the probable phase and composition of trapped hydrocarbons can be improved by taking hydrodynamic conditions and associated hydrocarbon migration conditions into account. For example: the changing pressure and temperature conditions, which influence the
246
Chapter 8
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IS
downdip
Figure 8.8 Cross-section of isotropic carrier-reservoir rock showing potential oil migration directions and trapping positions for different groundwater flow conditions.
Quantitative analysis of secondary hydrocarbon migration systems
247
Figure 8.9 Map showing potential oil trapping position as determined by the WZ mapping procedure (z = structure contours of top carrier-reservoir rock; vo = groundwater equipotential lines; uo = oil equipotential lines (from Dahlberg, 1982. Reprinted by permission of SpringerVerlag).
hydrocarbon phase and composition during migration, can be estimated from the identified lengths and directions of the hydrodynamic hydrocarbon migration paths; the previously determined trap types and their positions along the migration path influence the composition of hydrocarbons accumulating in successive traps; the probable occurrence and intensity of water washing and biodegradation along the migration path and during filling of the traps can be inferred directly from knowledge on the prevailing groundwater flow systems (Chapter 5). The identified characteristics of secondary hydrocarbon migration can be verified with e.g. the location and physico-chemical characteristics of known hydrocarbon accumulations (e.g. England and Mackenzie, 1989); direct and indirect observations of oil and gas seeps (direct visual observations; indirect observations, such as hydrocarbon-charged sediments, pock marks, clay diapirs); and gas leakages indicated by seismic chimneys.
Chapter 8
248
Different modelling techniques a r e available t h a t can simulate hydrodynamic hydrocarbon migration quantitatively on a basin-wide scale. For example, the Themis migration model simulates separate phase hydrocarbon migration under hydrodynamic conditions. This model i s basically a n extension of the groundwater flow model (Section 8.2.2) through the application of a n adapted two-phase Darcy flow equation (e.g. Doligez et al., 1987; Ungerer et al., 1987a, 1990). Garven (1989) simulated numerically hydrocarbon migration in aqueous solution and in separate phase in a gravity-induced groundwater flow system. Hydrocarbon migration models can be applied to study the role of t h e different parameters affecting migration a n d accumulation of hydrocarbons (e.g. England and Fleet, 1991).
Input “uid-flow data file Digitized section, paleobathy metries,
Compaction laws (effective stress /porosity)
output: Pressures Porosities + Hydraulic head
Input thermal data file
Remesblng file Sedlmentatlon rates
and two-phase flow
output: Transformation ratio
Input hydrocarbon generation and migration data file Kinetic parameters Initial organic matter distribution Relative permeability coefficients HC density and viscosity
(distribution and
Fluid velocities for hydrocarbons and
Figure 8.10 Example of the general organization of a n integrated basin model. The model is organized into five main modules: backstripping, h e a t transfer, single-phase fluid flow, hydrocarbon generation kinetics, and two phase migration These modules may be used separately or together. The four main input data files are 1. geological knowledge of section; 2. single-phase fluid flow; 3. heat transfer, and 4. geochemical data and physical parameters of two-phase migration. (from Ungerer e t al., 1990. Reprinted by permission of the American Association of Petroleum Geologists).
Quantitative analysis of secondary hydrocarbon migration systems
249
8.4 History of hydrocarbon migration systems
A quantitative basin evaluation for petroleum exploration requires a quantitative integrated analysis of the four time-dependent processes (generation, migration, accumulation, and preservation of hydrocarbons) that determine the present hydrocarbon potential of the basin. Computer-aided integrated basin modelling on the basis of numerical simulation provides means to reconstruct these processes and their interactions in time and space on a basin-wide scale. The development of numerical models reconstructing hydrocarbon generation and migration started end -70s (Durand et al., 1984; Ungerer et al., 1984;Welte and Yukler, 1980,1981).A large variety of models has been developed since (Anonymous, 1991;Doligez, 1987;England and Fleet, 1991; Poelchau and Mann, 1989). Two- and three-dimensional models that integrate migration models into comprehensive basin models, reconstruct the tectonic, structural and sedimentological evolution, together with the thermal evolution, the evolution of groundwater pressures, single-phase groundwater flow, generation and expulsion of hydrocarbons and two-phase migration of hydrocarbons and groundwater (Figure 8.10). Computer-aided integrated basin models reconstructing hydrocarbon migration at a basin-wide scale, are used t o increase the understanding of hydrocarbon migration systems and to increase exploration efficiency. As yet, different authors question the use of migration modelling as a stand-alone exploration tool providing reliable quantitative estimates of the volumes and composition of petroleum in traps, because of the uncertainties in parameters and processes, the nonuniqueness of the models and the quality, quantity and distribution of input data (e.g. B u m s et al., 1991; England et al., 1987;Mackenzie and Quigley, 1988;Schowalter, 1991). Important applications of integrated basin modelling include (e.g. Anonymous, 1991; England and Fleet, 1991) - the quantitative evaluation of the consequences of different hypotheses concerning interacting processes of importance for hydrocarbon migration and accumulation; - the quantitative evaluation of the consequences of different boundary and initial conditions on hydrocarbon migration and accumulation; - the determination of the type of parameters the hydrocarbon migration system is sensitive t o (e.g. physico-chemical properties of migrating hydrocarbons, permeability distribution, relative permeabilities; Figure 8.6); - the estimation of the quantity and the nature of hydrocarbons in traps.
CHAPTER a QUANTITATIVE ANALYSIS OF SECONDARY HYDROCARBON MIGRATION SYSTEMS
A quantitative analysis of secondary hydrocarbon migration systems should result in figures for the volumes and compositions of hydrocarbons migrating in a sedimentary basin as a function of time and space. Ideally, all aspects of a secondary hydrocarbon migration system in a sedimentary basin at a certain time during the basin’s evolution, should be quantified in the analysis, i.e. the masses and initial composition of hydrocarbons available for secondary migration, the three-dimensional migration pattern, the flux of migrating hydrocarbons and the migration losses. Different procedures are available for the quantitative determination of the masses and initial compositions of hydrocarbons available for secondary hydrocarbon migration (e.g. Duppenbecker et al., 1991;Mackenzie and Quigley, 1988;Tissot and Welte, 1984). The reader is referred to the published literature for a detailed outline of these procedures. This chapter presents approaches for the quantitative determination of the remaining characteristics of present and past secondary hydrocarbon migration systems. Emphasis is placed on the identification and integrated analysis of a wide variety of observable present-day physico-chemical characteristics of fluids and rocks in a sedimentary basin in order to quantify present-day migration characteristics and t o place constraints on the reconstruction of the history of hydrocarbon migration systems in the basin.
A quantitative analysis of present-day secondary hydrocarbon migration for basin evaluation can be restricted to the prospective parts of a sedimentary basin as selected on the basis of the previously described qualitative study (Chapter 7). The quantitative assessment of present-day hydrocarbon migration systems is described separately for hydrostatic and hydrodynamic conditions of the prospective parts of the basin (Sections 8.1,8.2and 8.3). Section 8.4 briefly describes the available approaches for a quantitative analysis of the evolution of secondary hydrocarbon migration systems.
Chapter 8
MaD view
A'
m .-..-. *
; -.*
Cross section
a
:
Structurecontours top carrier rock Hydrocarbon expelling source rocks Catchmentarea
A'
Hydrocarbon expelling source rock Drainagevolume
Figure 8.1 Map view and cross-section of hypothetical prospective area.
Quantitative analysis of secondary hydrocarbon migration systems
229
8.1 Present-day hydrostatic hydrocarbon migration systems In order t o quantify the separate phase hydrocarbon migration under true or assumed hydrostatic conditions, the location of hydrocarbon expelling source rocks, the amount and characteristics of expelled hydrocarbons and the basin's hydrogeological framework should be known. The hydrostatic separate phase hydrocarbon migration starts in the porous and permeable hydrogeological units (i.e. carrier-reservoir rocks) adjacent to the expelling source rocks. As outlined in Chapter 4, secondary hydrocarbon migration in hydrostatic basins is a preponderantly lateral migration through carrier-reservoir rocks. After expulsion from the source rock, hydrocarbons will move updip along the upper boundary of the adjacent carrier-reservoir rocks until traps are encountered. The migrating hydrocarbons will seek the shortest possible migration paths. In order to establish the hydrocarbon migration pattern, the geometry of the upper boundary of these carrier reservoir rocks and the location of permeable and impermeable zones along this boundary should be derived from the known hydrogeological framework of the basin (Section 6.3.4). The hydrocarbon migration pattern from source rock to potential trapping positions can be constructed with the help of a depth-contour map of the geometry of the upper part of the appropriate carrier-reservoir rocks (Section 7.1.2). The geometry of the upper boundary of the carrier-reservoir rock and the location of vertical seals along the migration path determine the possible trapping locations. After having identified the potential hydrocarbon migration paths from expelling source rock to a trapping position, the total volume of hydrocarbons lost WL) along the migration pathways can be estimated from the total volume of rock through which the hydrocarbons migrate (the drainage volume VD) and the mean porosity of that rock (n) (Section 4.3.2, Equation 4.25: V, = n&VD; S, = apparent residual saturation, estimated at 1 - 3%) as proposed by Mackenzie and Quigley (1988; Figure 8.1). Knowing the volume of petroleum expelled from that part of the source rock that provides a drainage area for the trap being evaluated, the total hydrocarbon charge that potentially is available for the trapping location can then be estimated from the difference between the volume of hydrocarbons expelled from the source rock and the volume of hydrocarbons lost during secondary migration (Mackenzie and Quigley, 1988). Repeating this procedure for all potential trapping positions along the identified migration pathways in the studied part of the basin, the potential trapping locations can be ranked according to hydrocarbon charge. The volume of hydrocarbons that actually have reached a certain trapping position in a presently stable hydrostatic basin is, in theory, also influenced by the time required for the hydrocarbons to reach the trap in relation t o the time
Chapter 8
230
elapsed since hydrocarbon expulsion from the source rock started to the present-day. The updip lateral migration of oily hydrocarbons proceeds at a specific discharge rate in the order of millimetres per year (Section 4.1) corresponding to velocities of tens of centimetres per year. Hence, the specific discharge rate will probably not be a limiting factor on the hydrocarbon charge for the generally encountered migration distances of less than 30 km (Section 4.3). Quantitative information on migrating hydrocarbons can be obtained by using the specific discharge equations given in Section 4.1. For example, the specific discharge for hydrostatic updip migration can be calculated from the equations
-2
r
khc = T n S h c
8Z
To solve these equations, requires knowledge on geometrical and hydraulic properties of the rocks (Section 6.3.4), densities of groundwater, and densities and viscosities of hydrocarbons. The density of the groundwater can be estimated from Figure 1.3 for different temperatures and salinities. As a first approach, the densities and viscosities of hydrocarbons estimated from, respectively, Figure 4.7 and Table 4.2 can be used in the calculations. The reader is referred to England et al. (1987) and Mackenzie and Quigley (1988) for more precise techniques to determine the density and viscosity of hydrocarbons for various subsurface conditions and for different compositions of the hydrocarbons. Different modelling approaches simulating hydrostatic hydrocarbon migration have been developed (Lehner et al., 1987; Sylta, 1987, 1991a, 1991b). Figure 8.2 shows the result of an example calculation, as given by Lehner et al. (19871, for hydrostatic separate phase hydrocarbon migration through a hypothetical carrier rock of indefinite thickness at different times during subsidence. It has been assumed in the calculation that the location of hydrocarbon input into the secondary migration system shifts with continued subsidence. Sylta (1991a) incorporated modelling of phase behaviour in the modelling of secondary hydrocarbon migration by using an equation of state of a multicomponent hydrocarbon mixture. Sylta’s modelling approaches also account for migration losses (Sylta, 1987, 1991).
Quantitative analysis of secondary hydrocarbon migration systems
--
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232
Chapter 8
8.2 Present-day hydrodynamic conditions
The quantitative analysis of hydrodynamic conditions in a prospective part of a basin includes the identification of the different types of groundwater flow system (gravity-induced, burial-induced, tectonically-induced), the quantitative assessment of the characteristics of the identified groundwater flow systems, the interaction and genetic explanation of the flow systems. For this purpose, a quantitative integrated analysis of direct and indirect indicators of regional groundwater flow is used in combination with groundwater flow modelling techniques. Section 6.3 provides information on the data sources for the principal direct and indirect indicators of flow (groundwater pressure; and temperature, salinity and chemical composition of groundwater). Irrespective of the type of sedimentary basin under consideration, i.e. the type of groundwater flow system under consideration, the current direction and intensity of groundwater flow at each point in a basin under isothermal and isochemical conditions are directly related to the groundwater potential gradient, the density and viscosity of groundwater and the permeability of the subsurface (Chapter 1). On a regional scale, the combination of groundwater pressure, density and viscosity data and permeability data directly indicate e.g. the lateral groundwater flow pattern, areas of concentrated horizontal or vertical flow and the location of geopressured zones. Some characteristics of indirect indicators of flow may be associated with a particular flow condition independent of the type of flow system under consideration (e.g. positive anomalies of groundwater temperatures and salinities at shallow depths may indicate focussed vertical upward flow of groundwater). A correct genetic interpretation of the direct and indirect indicators of groundwater flow and the associated groundwater flow directions and velocities requires knowledge on the type and evolution of the groundwater flow systems involved. The previously described qualitative study is a first approach to differentiate between the different groundwater flow systems (Chapter 7). 8.2.1 Hydrodynamic conditions in stable subaerial regions The hydrodynamic condition in a stable subaerial region is given by the characteristics of the prevailing gravity-induced groundwater flow systems.
The present-day groundwater flow systems in the selected study area may not be in accordance with the relief of the present-day water table, and as a consequence the characteristics of the flow system cannot be reliably inferred solely from water table relief and subsurface permeability distribution. A direct determination of the quantitative characteristics of the present-day gravityinduced groundwater flow system, requires data on pressure, density and viscosity of groundwater and data on the permeability distribution
Quantitative analysis of secondary hydrocarbon migration systems
233
supplemented with data on the relief of the present-day water table, o r eventually on the topographic relief of the ground surface. The interpretation of these data should be integrated with a n evaluation of the different characteristics associated with gravity-induced groundwater flow, as apparent at the ground surface (moisture conditions) or in the subsurface (temperature, salinity, hydrochemical composition, isotopic composition of groundwater) (Chapter 2). T6th (1978) proposed an analysing technique for the quantitative assessment of cross-formational gravity-induced groundwater flow systems and the genetic explanation of the identified systems, based on the integrated interpretation of the following five pressure-related parameters: - potentiometric surface - pressure-depth relation - dynamic pressure increment - hypsographic distribution - water table elevation.
Potentiometric surface The groundwater potentials of a certain hydrogeological unit can be calculated from groundwater pressure and density data (Chapter 1). Groundwater potentials are often expressed in equivalent fresh-water heads. Equipotential lines, or potential contours, connect points of equal groundwater potential for a single hydrogeological unit. The potential contours for a hydrogeological unit add up to a relief map of the potentiometric surface for that unit. A potentiometric surface map of a hydrogeological unit provides a regional picture of the magnitudes and directions of the groundwater potential gradients, which is also of direct importance in analysing hydrocarbon migration systems. For analysing groundwater flow conditions, the direction and spacing of the potential contours of the potentiometric surface of a hydrogeological unit can be used to identify the lateral component of the groundwater flow pattern, the location of lateral barriers to flow andor the location of vertical escape ways from the unit. Pressure -depth relation A regional picture of groundwater pressure-depth relations is indicative of the following groundwater flow characteristics: groundwater flow condition (hydrostatic or hydrodynamic), groundwater flow direction (descending, horizontal or ascending), absence or presence of pressure barriers. Dynamic pressure increment The dynamic pressure increment, Apw, at a certain depth is the difference between the hydrostatic and the hydrodynamic pressure-depth relation at that depth (T6th, 1978). T6th (1978) showed that the dynamic pressure increment is a function of both ground surface elevation and depth of measurement. He proposed the use of a two-dimensional presentation of this relation (the elevation-depth pattern: Apw-z-d)as an analysing tool. The elevation-depth
Chapter 8
234
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'
D
r 4 4 10000~ a
3
L i n e o f equal dynamic p r e s s u r e increment
from T6th,
1978, F i g .
Figure 8.3 Theoretical distribution of hydraulic head, groundwater flow, pressure and dynamic pressure increment in a homogeneous drainage basin with simple ground surface geometry (from T6th, 1980. Reprinted by permission of the American Association of Petroleum Geologists).
Quantitative analysis of secondary hydrocarbon migration systems
235
LEGEND c'
Regional s l o p e
a
Amplitude o f l o c a l topography
,---Line
o f equal h y d r a u l i c
head
,
,
Line o f flow
c -
Boundary between f l o w systems
(Adapted f r o m T 6 t h , 1963, F i g . 2 f , p. 4802)
P Supemorma 1 pressures
LEGEND + A P ~ Dynamic p r e s s u r e increment a t s i t e 4, d e p t h 8000 f t a
Local system
b
I n t e r m e d i a t e system
c
Regional system
. i n e o f equal oynamic Nressure increment from T6th,
1978, F i g .
a
Figure 8.4 Theoretical distribution of hydraulic head, groundwater flow, pressure and dynamic pressure increment in a homogeneous drainage basin with complex ground surface geometry (from Tdth, 1980. Reprinted by permission of the American Association of Petroleum Geologists).
pattern i s a family of is0 Apw curves plotted against groundwater head elevation, z, and depth of measurement, d (Figures 8 . 3 ~and 8.44. Such an elevation-depth pattern of the dynamic pressure increment can be used to identify the type of gravity-induced flow systems (local, intermediate, regional
Chapter 8
236
flow systems), the lateral and vertical extent of each flow system, the hydrogeological framework, the hydraulic continuity between separate porous and permeable hydrogeological units, and the effect of ground surface topography on the groundwater pressure distribution.
elevation, arbitrary units
elevation, arbitrary units
,.:
0
25
50
75
100%
confined aquifer
A,]'
a
basin's ground surface
s'
hypsographic curve of basin's ground surface
Reqionally confined aqulfer Shape and position of groundwater potentiometric surface are independent of coafiguration shown portion of basin's ground surface
75
100%
h,
potentiometric surface h,
.........._. ....h i
hypsographlc curve of potentlometric surface hl
V
unconfined aquifer
s
50
relative extent of study area
relative extent of study area -A,
25
0
b
Reqionally unconfined aqulfer Shape and position of groundwater potentiometric surface are dependent of configuration shown portion of basin's ground surface
Figure 8.5 Relation between hypsographic curve of the ground surface and t h a t of the groundwater potentiometric surface of regionally confined a n d unconfined aquifers (after T6th, 1978, Water Resources Research, Vol. 14,no. 5, Fig. 7, p. 813, Copyright (0)1978 by the American Geophysical Union).
Quantitative analysis of secondary hydrocarbon migration systems
237
Hypsographic distribution A hypsographic distribution is the relative areal frequency in a certain region of elevations above datum of a laterally extensive bounded surface. It can be expressed by the hypsographic curve, which is a plot of elevations representing specified vertical intervals, e.g. of the ground surface against the cumulative sums of the relative extent of the corresponding horizontal projections. The analysing technique proposed by T6th is based on a comparison of the hypsographic curves of the ground surface and a potentiometric surface of a particular region. The analysing technique may be used t o determine whether a particular part of the groundwater flow system is in accordance with present ground surface topography. Figure 8.5 gives an example of a theoretical hypsographic distribution. According to T6th (1978) this technique can be very useful in large areas with varied topography and sparse data control. Water table elevation The elevation of the water table is the level where the groundwater pressure is atmospheric, i.e. the groundwater in an observation well would rise t o this level. The elevation of the water table is the vertical distance between a datum plane and this level, which corresponds to the water table within the aquifer (phreatic surface) or to the potentiometric surface. The elevation of the water table may be derived from groundwater pressure-depth plots (T6th, 1978). This parameter may be useful for determining outcrop regions of hydrogeological units and for identifying whether the groundwater flow is regionally unconfined or not, by correlating the elevation of the water table with the elevation of the ground surface.
The interpretation of the five pressure-related parameters can be supported by an evaluation of physico-chemical phenomena associated with gravityinduced groundwater flow and a comparison of present-day groundwater flow characteristics with theoretical flow characteristics for different hypothetical conditions and/or paleogeological and paleohydrogeological conditions. The correctness of the identified characteristics of the groundwater flow systems can be verified by analysing the observed physico-chemical characteristics of the studied region in the same way as outlined in Section 7.1.3.1. In addition, the regional distribution of indirect indicators of flow, especially temperatures, may be used t o identify geohydrological characteristics of the basin, such as directions and fluxes of groundwater flow and permeability distributions. For example, Chapman et al. (1991) used temperature distributions to constrain their numerical simulations of coupled groundwater flowheat flow for the Uinta Basin, USA. By varying geohydrological and thermal parameters they could estimate basin-scale permeabilities and groundwater flow conditions (see also Willet and Chapman 1987,1989;Smith et al., 1989).
238
Chapter 8
For the genetic interpretation of the five pressure-related parameters the observed distribution of parameter values may be compared with the theoretical distribution of parameter values as calculated for hypothetical models. T6th (1978, 1979, 1980) applied this approach in his regional groundwater flow study of the Red Earth Region in Alberta, Canada. Figures 8.3, 8.4 and 8.5 present examples of calculated parameter distributions for two different theoretical situations as given by T6th (1978,1979, 1980). Figures 8.3 and 8.4 show the distribution of the following parameters in a hypothetical homogeneous and hydraulically continuous basin with simple and complex topographic relief, respectively: groundwater potential and groundwater flow ( Figures 8.3a and 8.4a); groundwater pressure (Figures 8.3b and 8.4b) and dynamic pressure increment (Figures 8 . 3 ~ and 8.4~).An example of a theoretical hypsographic distribution is given in Figure 8.5. The quantitative characteristics of the gravity-induced groundwater flow systems identified by the interpretation of the five pressure-related parameters can be explained genetically by applying an appropriate simulation model of gravity-induced groundwater flow (e.g. Bethke, 1986a, 1989; Garven, 1989; Garven and Freeze, 1984; Zijl, 1984, 1988, 1989). Assuming steady-state conditions in the area of study, simulation of gravity-induced flow systems resulting from the present-day water table potential distribution and subsurface permeability distribution, gives information on the geometry of the flow systems, the groundwater flow directions and velocities and the groundwater potential distributions. By comparing the identified characteristics of the groundwater flow systems with those calculated for the present-day relief of the water table and the present-day permeability distribution, the characteristics of the present-day flow systems that are in accordance with the present-day relief of the water table can be identified. The maximum depth of a steady-state groundwater flow condition can thus be estimated as well. The present-day groundwater potential distributions and groundwater flow directions in the deeper parts of the basin may still be related t o former configurations of the water table (i.e. former topographic, climatic and/or sea level conditions; Section 2.3). The interpretation of such transient hydrodynamic conditions requires data on the paleotopography of the groundsurface, or on paleoclimatic and -sea level conditions, and data on the paleohydrogeological framework. Past water table configurations may be inferred from knowledge on the recent geological evolution of the basin. The reconstructed past water table configurations and the known or assumed paleohydrogeological framework of the basin may be used as input data for a steady-state groundwater flow simulator t o determine the characteristics of past groundwater flow systems in accordance with the corresponding past water table configurations. A combination of such steady-state simulations of past gravity-induced groundwater flow conditions with calculations of adjustment times of groundwater potential t o changing boundary conditions can be used to interpret the present-day transient groundwater flow conditions
Quantitative analysis of secondary hydrocarbon migration systems
239
(e.g. Garven 1989; T6th and Corbet, 1987). Simulation of the hydrogeohistory by applying models of unsteady-state gravity-induced groundwater flow are also used to interpret present-day transient groundwater flow condtions (e.g. Senger et al.,1987). 8.2.2 Hydrodynamic conditions in subsiding and filling basins
The present-day burial-induced hydrodynamic conditions in the selected area can be assessed directly from the groundwater pressure, density and viscosity data and the permeability data. Each subsystem of burial-induced groundwater flow is characterized by specific vertical and horizontal changes of the groundwater potential (Section 2.1.3): Characteristics of the shallow subsystem: the groundwater potential increases slightly with depth, reflecting cross-formational vertical upward flow of groundwater; there is no lateral change in groundwater potential; the groundwater pressure-depth gradient is near hydrostatic t o slightly superhydrostatic. Characteristics of the intermediate subsystem: the groundwater potential in fine-grained rocks is higher than that in adjacent relatively coarse-grained rocks; the groundwater potential in the relatively coarse-grained rocks changes laterally, indicating the lateral flow direction of groundwater through these rocks; the groundwater pressures in the relatively coarse-grained rocks are superhydrostatics; the groundwater pressure-depth profile in the relatively coarse-grained rocks runs parallel to the hydrostatic gradient. Characteristics of the deep subsystem: the fine-grained as well as the coarsegrained rocks are geopressured; large vertical changes in groundwater potential occur over the fine-grained rocks and large lateral changes in groundwater potential occur in the relatively coarse-grained rocks over relatively short distances, reflecting the restricted groundwater flow conditions in the deep subsystem. A regional picture of changes in groundwater pressure with depth and groundwater potential with depth in combination with potentiometric surfaces constructed for different hydrogeological units permit the delineation of the vertical and lateral extent of the shallow, intermediate and deep subsystems of burial-induced flow in the studied area.
Subsequently, additional characteristics of the burial-induced hydrodynamic conditions of importance for hydrocarbon migration, such as directions and rates of groundwater flow, zones of concentrated flow, should be determined for each of the subsystems. The shallow subsystem of burial-induced flow can be considered to be hydraulically continuous. Therefore, the concept of dynamic pressure increment, as introduced by Tdth (1978; Section 8.2.1) can be used to
240
Chapter 8
assess the vertical direction and rate of groundwater flow in the shallow subsystem. The pattern of lateral groundwater flow through the relatively permeable hydrogeological units in the intermediate subsystem can be derived from the potentiometric surface maps of the groundwater constructed for each of these units. It is also important to look for discontinuities in the distribution of the groundwater potentials (such as large differences of potential over short lateral distances and, conversely, zones of very small lateral differences of potential) in relation t o tectonic and structural elements or stratigraphic features, because they may indicate the location of lateral barriers t o flow andlor the location of vertical escape ways for groundwater from the hydrogeological unit. The rates of lateral groundwater flow through the relatively permeable hydrogeological units can be calculated from the lateral groundwater potential gradients, the groundwater density and viscosity and the permeabilities of the hydrogeological unit. The magnitudes and directions of the changes in groundwater potential with depth in the poorly permeable units in the intermediate subsystem affect the sealing capacity of these units for hydrocarbons. Ideally, a detailed regional picture of groundwater potential profiles over these units should be constructed from pressure data o r pressure indicators (Section 6.3). The potentiometric surface map of the groundwater in each of the relatively coarse-grained units in the deep geopressured subsystem in combination with lithostratigraphic and structural information of these units will indicate the lateral direction of groundwater flow, location of vertical barriers t o flow, and the location of possible vertical escape ways for groundwater. The regional distribution of the magnitudes of groundwater pressures and potentials in the geopressured zone will help t o delineate possible zones of seal failure resulting from hydraulic fracturing. Zones of concentrated upward flow of groundwater, which may occur along the edges of an intermediate subsystem, along permeable faults and diapirs, and through hydrofractured zones of seal failure in the deep geopressured subsystem, induce positive pressure, temperature and salinity anomalies and associated mineral assemblages in the shallow parts of the basin. The location of the zones of concentrated upward flow of groundwater inferred from the interpretation of groundwater potential and pressure data may thus be confirmed by an evaluation of available thermal and geochemical information (see Chapter 7). The identified pressure distribution and the other physico-chemical characteristics associated with burial-induced groundwater flow are the combined result of the time-dependent processes of groundwater pressure generation and dissipation. The groundwater pressures in a filling sedimentary basin are generated by the combined effect of the increase in load of the water-saturated sediments and the aquathermal effects, which, a t greater depths, may be enhanced by the dehydration of clay minerals and by hydrocarbon generation from organic matter (Section 2.1.3). Ideally, a
Quantitative analysis of secondary hydrocarbon migration systems
241
quantitative genetic interpretation of the identified present-day hydrodynamic conditions should take into account the combined influence of the different pressure-generating mechanisms and the hydrogeological framework of the basin. Different theoretical approaches for establishing groundwater pressure distributions in a subsiding and filling basin have been presented (Bethke, 1985, 1986b;Bethke and Corbet, 1988;Bethke et al., 1988;Doligez et al., 1986; Forbes et al., 1992; Magara, 1978, 1986, 1987; Mann and Mackenzie, 1990; Ungerer et al., 1987b). These approaches include two-dimensional basin-scale modelling techniques to simulate groundwater pressure generation and groundwater flow (e.g. Bethke’s model, Themis model). Bethke’s model (1985, 1986) considers the development of groundwater pressures and groundwater flow in a subsiding and filling basin as resulting from compaction, aquathermal pressuring and mineral dehydration reactions (Bethke, 1985,1986; Bethke et al., 1988; Harrison and Summa, 1991). Using Bethke’s model, Bethke et al. (1988) and Harrison and Summa (1991) simulated and interpreted the development of present-day groundwater pressures and groundwater flow in the Gulf of Mexico Basin, USA (Figure 2.30). The Themis (Temispack) model is a twodimensional model, which can simulate numerically compaction and burial processes, hydraulic fracturing, groundwater flow, conductive and convective heat transfer and hydrocarbon generation by using Darcy’s law for groundwater flow, porosity versus effective stress relationships for compaction, parallel first order kinetic reactions and Arrhenius laws for hydrocarbon generation (Doligez et al., 1986; Forbes et al., 1992; Ungerer et al., 1987b, 1990). The Themis model has been used to study hydrodynamic conditions in, for example, the North Sea Basin (Figure 8.6; Bunus et al., 1991; Doligez et al., 1987; Ungerer et al., 1987a1, the Mahakam Delta, Indonesia (Burrus et al., 1991; Schneider et al., 1991), the Eastern Canadian margin (Forbes et al., 1992) and the Paris Basin (Burrus et al., 1991). The application of the above-mentioned modelling techniques allow the assessment of the relative importance of the possible pressure generating mechanisms and the different characteristics of the hydrogeological framework in relation to the observed present-day hydrodynamic conditions in the basin. 8.2.3 Hydrodynamic conditions resulting from interactions of
different groundwater flow systems Different groundwater flow systems may co-exist and interact in a basin (Section 2.5).
When a subsiding basin is surrounded by continental areas or when part of the basin has emerged and stabilized above sea level, the hydrodynamic characteristics of such basins reflect the interaction of burial- and gravity-
Chapter 8
242
a
Krn
Krn
-
-
AGE . 000 M.A.
Krn
W a t e r velocity ( I 0 to GOOm/MA.)
AGE OOOMA
Krn Water velocity ( I 0 t o GOOm/MA.)
Figure 8.6 Temispack reconstruction of the present-day distribution of overpressures induced by compaction disequilibrium for two assumed conditions of fault permeability: a. faults are assumed to be permeable; b. faults are assumed t o be impermeable (arrows: Darcy velocity) (from Burrus e t al., 1991, Geological Society Special Publication no. 59, Fig. 7, p. 97. Reprinted by permission).
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induced flow systems. Features, such as positive pressure, temperature and salinity anomalies are characteristic of zones of concentrated upward flow along the edges of subsiding basins as well as of upward flow in discharge zones of gravity-induced flow systems. Along the edges of subsiding basins surrounded by continental areas, these features may be genetically related to either one of these flow systems or to both. The genetic interpretation of the observed direct and indirect indicators of groundwater flow in the basin should include an evaluation of both flow systems and their interactions. Bethke et al. (1988) and Harrison and Summa (1991) used two-dimensional groundwater flow modelling techniques taking into account the two driving forces for groundwater flow, i.e. sedimentation in the subsiding part of a basin and relief of the groundwater table in its continental part, to reproduce present-day hydrodynamic conditions in the Gulf of Mexico Basin. Burrus et al. (1991) showed numerical results of a version of the Themis model that includes the two driving forces for groundwater flow for the Paris Basin and the Mahakam Delta. Hydrodynamic conditions in a basin may in part result from tectonic forces. Ge and Garven (1989) applied a numerical model of coupled tectonic- and gravity-induced flow to evaluate the relative importance of tectonic influence on groundwater 'pressure and flow in an otherwise gravity-induced flow system in a hypothetical foreland basin. Forbes et al. (1992) included an evaluation of lateral compression in their numerical reconstruction of the present-day pressure distribution in the Venture Field, Eastern Canada. Free thermal and thermohaline convection may occur locally in sedimentary basins. The possible occurrence of thermal convection of groundwater should be evaluated in sedimentary basins with high heat flows and around magmatic intrusions and salt diapirs. The possible existence of thermohaline convections should be evaluated near evaporite occurrences (Section 2.4).
8.3 Present-day hydrodynamic hydrocarbon migration systems The identified types of groundwater flow system, their groundwater-depth gradients and potentiometric surfaces in combination with the hydrogeologic _",-&meworkallow an evaluation of the sealing capacity of the barrier rocks under hydrodynamic conditions (e.g. lateral continuity of barrier rocks of sufficient sealing capacity; zones of seal failure by hydraulic fracturing of rocks) and the role of faults as vertical pathways or barriers for hydrocarbon migration (Sections 5.2 and 5.3).
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’,.
I
/
.I
-
Figure 8.7 Cross-section showing graphical relation between equipotential surfaces, u = constant, and those of v = constant and z = constant for equal intervals of Au = Av = Az (after Hubbert, 1967).
The migration pattern for hydrocarbons in aqueous solution and very fine suspension can be derived directly from the previously identified groundwater flow patterns.
For parts of the basin with laterally continuous barrier rocks of sufficient sealing capacity, the pattern of separate phase hydrocarbon migration through carrier-reservoir rocks and the location of trapping positions can be determined by applying Hubbert’s mapping technique (Dahlberg, 1982;Hubbert, 1953,1967) assuming that the locations of hydrocarbon expelling source rocks are known. The application of Hubbert’s W Z mapping procedure requires information on the geometry of the upper boundary of the carrier-reservoir rocks, the groundwater potential distribution in the carrier-reservoir rocks, the groundwater density and the hydrocarbon density. Section 5.2 showed that the potential of a unit mass of separate phase hydrocarbons in a water-saturated rock under hydrodynamic conditions is given by Equation 5.2. Omitting the influence of capillary pressures on the hydrocarbon potential reduces Equation 5.2 to (8.i) Equation 8.1 is the basis of Hubbert’s mapping procedure. For constant groundwater and hydrocarbon densities, Equation 8.1 can also be expressed as
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2A5
(8.2) By letting
it follows that Uhc = vhc - z. At every point in a carrier-reservoir rock, Uhe, which is proportional to the hydrocarbon potential, can be determined from the elevation z and the value of Vhc, which can be calculated from the groundwater potential (Figure 8.7). The UVZ mapping procedure results i n maps o r cross-sections showing hydrocarbon equipotential surfaces in carrier-reservoir rocks from which hydrocarbon migration directions and potential trapping positions can be derived (Figures 8.8 and 8.9). The volume of hydrocarbons lost along a migration path from expelling source rock t o potential trapping position, can be estimated by applying Mackenzie and Quigley's procedure (Section 8.1). Under hydrodynamic conditions, additional losses can be expected to occur by removal of light hydrocarbons in aqueous solution. After having estimated the hydrocarbon charges available for the different trapping locations, the traps can be ranked according to hydrocarbon charge. The calculation of specific discharge rates for separate phase hydrocarbon migration requires knowledge on the geometrical and hydraulic properties of the carrier-reservoir rocks (Section 6.3.41,the densities of groundwater, the densities and viscosities of hydrocarbons (Section 8.1) and the groundwater potential gradients in the carrier-reservoir rock. Specific discharge rates for separate phase hydrocarbon migration under hydrodynamic conditions can be calculated from the following equations given in Sections 4.1 and 4.2
An evaluation of the probable phase and composition of trapped hydrocarbons can be improved by taking hydrodynamic conditions and associated hydrocarbon migration conditions into account. For example: the changing pressure and temperature conditions, which influence the
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a
Groundwater flow direction
b
Groundwater flow direction is downdip Groundwater flow velocity b < a
c
Groundwater flow direction is updip no potential oil trapping position
IS
downdip
Figure 8.8 Cross-section of isotropic carrier-reservoir rock showing potential oil migration directions and trapping positions for different groundwater flow conditions.
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Figure 8.9 Map showing potential oil trapping position as determined by the WZ mapping procedure (z = structure contours of top carrier-reservoir rock; vo = groundwater equipotential lines; uo = oil equipotential lines (from Dahlberg, 1982. Reprinted by permission of SpringerVerlag).
hydrocarbon phase and composition during migration, can be estimated from the identified lengths and directions of the hydrodynamic hydrocarbon migration paths; the previously determined trap types and their positions along the migration path influence the composition of hydrocarbons accumulating in successive traps; the probable occurrence and intensity of water washing and biodegradation along the migration path and during filling of the traps can be inferred directly from knowledge on the prevailing groundwater flow systems (Chapter 5). The identified characteristics of secondary hydrocarbon migration can be verified with e.g. the location and physico-chemical characteristics of known hydrocarbon accumulations (e.g. England and Mackenzie, 1989); direct and indirect observations of oil and gas seeps (direct visual observations; indirect observations, such as hydrocarbon-charged sediments, pock marks, clay diapirs); and gas leakages indicated by seismic chimneys.
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Different modelling techniques a r e available t h a t can simulate hydrodynamic hydrocarbon migration quantitatively on a basin-wide scale. For example, the Themis migration model simulates separate phase hydrocarbon migration under hydrodynamic conditions. This model i s basically a n extension of the groundwater flow model (Section 8.2.2) through the application of a n adapted two-phase Darcy flow equation (e.g. Doligez et al., 1987; Ungerer et al., 1987a, 1990). Garven (1989) simulated numerically hydrocarbon migration in aqueous solution and in separate phase in a gravity-induced groundwater flow system. Hydrocarbon migration models can be applied to study the role of t h e different parameters affecting migration a n d accumulation of hydrocarbons (e.g. England and Fleet, 1991).
Input “uid-flow data file Digitized section, paleobathy metries,
Compaction laws (effective stress /porosity)
output: Pressures Porosities + Hydraulic head
Input thermal data file
Remesblng file Sedlmentatlon rates
and two-phase flow
output: Transformation ratio
Input hydrocarbon generation and migration data file Kinetic parameters Initial organic matter distribution Relative permeability coefficients HC density and viscosity
(distribution and
Fluid velocities for hydrocarbons and
Figure 8.10 Example of the general organization of a n integrated basin model. The model is organized into five main modules: backstripping, h e a t transfer, single-phase fluid flow, hydrocarbon generation kinetics, and two phase migration These modules may be used separately or together. The four main input data files are 1. geological knowledge of section; 2. single-phase fluid flow; 3. heat transfer, and 4. geochemical data and physical parameters of two-phase migration. (from Ungerer e t al., 1990. Reprinted by permission of the American Association of Petroleum Geologists).
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8.4 History of hydrocarbon migration systems
A quantitative basin evaluation for petroleum exploration requires a quantitative integrated analysis of the four time-dependent processes (generation, migration, accumulation, and preservation of hydrocarbons) that determine the present hydrocarbon potential of the basin. Computer-aided integrated basin modelling on the basis of numerical simulation provides means to reconstruct these processes and their interactions in time and space on a basin-wide scale. The development of numerical models reconstructing hydrocarbon generation and migration started end -70s (Durand et al., 1984; Ungerer et al., 1984;Welte and Yukler, 1980,1981).A large variety of models has been developed since (Anonymous, 1991;Doligez, 1987;England and Fleet, 1991; Poelchau and Mann, 1989). Two- and three-dimensional models that integrate migration models into comprehensive basin models, reconstruct the tectonic, structural and sedimentological evolution, together with the thermal evolution, the evolution of groundwater pressures, single-phase groundwater flow, generation and expulsion of hydrocarbons and two-phase migration of hydrocarbons and groundwater (Figure 8.10). Computer-aided integrated basin models reconstructing hydrocarbon migration at a basin-wide scale, are used t o increase the understanding of hydrocarbon migration systems and to increase exploration efficiency. As yet, different authors question the use of migration modelling as a stand-alone exploration tool providing reliable quantitative estimates of the volumes and composition of petroleum in traps, because of the uncertainties in parameters and processes, the nonuniqueness of the models and the quality, quantity and distribution of input data (e.g. B u m s et al., 1991; England et al., 1987;Mackenzie and Quigley, 1988;Schowalter, 1991). Important applications of integrated basin modelling include (e.g. Anonymous, 1991; England and Fleet, 1991) - the quantitative evaluation of the consequences of different hypotheses concerning interacting processes of importance for hydrocarbon migration and accumulation; - the quantitative evaluation of the consequences of different boundary and initial conditions on hydrocarbon migration and accumulation; - the determination of the type of parameters the hydrocarbon migration system is sensitive t o (e.g. physico-chemical properties of migrating hydrocarbons, permeability distribution, relative permeabilities; Figure 8.6); - the estimation of the quantity and the nature of hydrocarbons in traps.