Burial and thermal history of Proterozoic source rocks in Oman

Precambrian Research, 54 ( 1991 ) 15-36

15

Elsevier Science Publishers B.V., Amsterdam

Burial and thermal history of Proterozoic source rocks in Oman Wiekert Visser Koninklijke/Shell Exploratie en Productie Laboratorium, Postbus 60, 2280 AB Rijswijk, The Netherlands (Received May 3, 1990; accepted after revision October 8, 1990)

ABSiRACT Visser, W., 1991. Burial and thermal history of Proterozoic source rocks in Oman. In: M.R. Walter (Editor), Proterozoic Petroleum. Precambrian Res., 54:15-36. A geological and thermal history of the Proterozoic sedimentary basin in Oman was reconstructed to determine the time of hydrocarbon generation from the Proterozoic Huqf Group source rocks. Data from 97 exploration wells, and supplementary seismic reflection profiles were used to establish the burial graphs for the source rocks at all 97 locations. The overburden removed during formation of regional unconformities was estimated from regional geological data, compaction trends, and fission track analyses. The thermal history was reconstructed on the basis of present-day temperature data in the wells, heat flow models, and the palaeo surface temperature history. The calculated present-day surface heat flow map shows very low values (35-40 m W / m 2) over the basin axis, while on the edges the values are in the range 55-65 m W / m 2. The results of the maturity calculations show that in northwest Oman oil generation occurred mainly during the Mesozoic, and that probably a large proportion of the traps in which reserves have been discovered was formed just before the last oil generation phase. However, in the central and'southeastern part of the country hydrocarbon generation stopped about 400 Ma ago. This is due to the maximum burial temperature being reached at around that time. Considering the fact that almost all traps in which reserves have been located are significantly younger than the time ofoil generation, one has to invoke processes of remigration to explain the oil occurrences. The geological data from Oman show that the eastern margin of the country, presently facing the Arabian Sea has been a high for most of its Phanerozoic history. The data on quantities of eroded and deposited sediments lend support for the theory that the margin was formed by a transform fault.

Introduction

The Sultanate of Oman is situated on the southeastern corner of the Arabian Peninsula. The country is currently producing some 600,000 bbls of oil per day from 50 fields. Geological literature to date has focussed on outcrops in the Oman Mountains (e.g. Glennieet al., 1974; Lippard et al., 1982), while information on the sedimentary basin hidden under the fiat desert plain has been very limited (e.g. Murris, 1981; Hughes Clarke, 1988 ). This paper is based mainly on information from hydrocarbon exploration well data, and its purpose is to discuss the thermal and burial history of the Proterozoic Huqf Group, the

source of most of the hydrocarbons discovered in Oman (Grantham et al., 1988). The Proterozoic sedimentary basin comprises most of the interior of Oman, south of the Oman Mountains (Fig. 1; Glennie, 1977 ). Its stratigraphy exibits a clear division into four main successions (Hughes Clarke, 1988; Fig. 2): (1) a carbonate platform and evaporite succession of the Proterozoic Huqf Group, (2) a clastic succession of the Cambro-Silurian Haima Group, (3) a terrestrial elastics succession and associated carbonate platform during the Late Carboniferous and Permian (Haushi and Akhdar Groups), and (4) a carbonate platform succession of the Mesozoic and Tertiary. Long periods of erosion and/or non-de-

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position occurred during the Devonian-Carboniferous, and Triassic-Lower Jurassic. It should be noted here that the stratigraphy and absolute age of the H u q f Group is still uncertain. The Upper H u q f Group Ara Formation could be of Early Cambrian, rather than Proterozoic age, but evidence is inconclusive. However, these uncertainties do not noticably influence the results of this source rock modelling study.

Source rock development Source rocks of the H u q f Group have been proven by well penetrations, and are ubiquitous within the Middle H u q f carbonates in south Oman. They occur mainly in the argilla-

ceous dolomitic Shuram Formation. In north Oman, where the Middle H u q f is beyond drillable depth, its source rock potential is uncertain, but is assumed to be similar to that in south Oman, based on the general similarity in seismic character. In the Huqf Axis outcrop area (Fig. 1 ) source rocks have not been encountered in beds that are supposed to be age equivalents of subsurface Middle H u q f source rock intervals (Gorin et al., 1982). Upper H u q f Group (Ara) source rocks have been proven by well penetrations in south Oman, but their distribution appears patchy. The source rock intervals may be embedded within the Ara salt, or may occur in a carbonate unit directly overlying the salt succession. Shaly siliceous source rocks directly overlying the Middle

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ferred to be present from a single well penetration, and from the occurrence o f p o s t m a t u r e source rocks scatterred in o u t c r o p s o f salt diapirs (e.g. Qarat A1 Kibrit, near well B, Fig. 4 ) .

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Structural history The structural development of the study area is characterised mainly by mild deformation within the basin, but at the margins major tectonic activity has shaped and reshaped the basin geometry (Fig. 3) and has influenced the sedimentation patterns during different geological periods. The extent of the basin at the time of deposition of the Middle Huqf platform carbonates is unknown. At the end of the Middle Huqf period a major phase of epi-cratonic rifting created the Upper Huqf (Ara) salt basins (Fig. 1 ). Of these, the Ghaba and South Oman Salt Basins both have retained evidence of their origin as rift basins, as can be seen on seismic reflection profiles (V. Carayon, pers. commun., 1987), while the Fahud Salt Basin seems to lack evidence for Cambrian rifting. The rifting event that created the Ghaba and South Oman Salt Basins can be correlated with the formation of other salt basins in the region (Husseini, 1988). Major strike-slip dominated faulting with a strong compressional element reshaped the western and southern margins of the South Oman Salt Basin at the end of the Huqf period (P. Diebold, pers. commun., 1989). Large-scale uplift in the region resulted in a massive influx of clastic sediments (Haima Group), which form an overall fining upward succession. Halokinesis during the Cambrian created salt-cored as well as turtle-back structures. By the Late Ordovician/ Early Silurian most of the relief in the region had been eroded away, and the Huqf Basin opened up northwestward, becoming part of the Rub' al Khali sedimentary basin. The end of the Haima depositional setting was caused by a period of basin-wide uplift, which was most pronounced along the eastern and northern edges of the basin, and which is thought to be related to a large scale doming, preceeding the break-up of Gondwana. Along the present east coast of Oman basaltic plugs outcrop, which have been dated 400-440 Ma. Sedimentation resumed with the Late Carboniferous

glacial/fluviatile deposits of the Haushi (Braakman et al., 1982). During the Permian, the Tethys margin was created in the north as evidenced, for example, by Permian turbidites in the Oman Mountains (Blendinger, 1989). This passive margin development gave rise to the extensive carbonate shelf development of the Akhdar Group, which spread over most of the epicontinental sea of the Rub'al Khali basin. "The (proto)Indian Ocean was created during the lower Mesozoic, but no evidence exists along the east coast of Oman for the development of a passive margin succession related to this new ocean. The results of this study support the theory that the break-up occurred along a large transform fault (Barton et al., 1990; Besse and Courtillot, 1988). Dissolution of Upper Huqf salt deposits caused structuring on the Eastern Hank of the South Oman Salt Basin during Jurassic, Upper Cretaceous, and Oligocene phases of regional structural development (Aley and Nash, 1984). During this period an extensive shallow epicontinental sea existed over all of Oman, with sediments generally thinning southeastward. By the end of the Cretaceous, closure of the Tethys ocean basin resulted in obduction of the Semail ophiolites in the north (Lippard et al., 1982; Glennie et al., 1974). This tectonic event also caused gentle deformation of the sediments south of the Oman Mountains, and the formation of traps of some of the main oil fields in north Oman. At about this time the Masirah ophiolites were emplaced along the Indian ocean margin, possibly along a strike-slip system (the Masirah line; Shackleton and Ries, 1988; Moseley, 1988 ). During the Oligocene, the Oman Mountains were uplifted, and the opening of the Gulf of Aden led to wrench faulting over large areas in Oman.

Hydrocarbon generation domains The structural development and hydrocarbon habitat modelling outlined in this paper suggest that interior Oman can be divided into

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ometry, and Devonian uplift created tilting along the east flank. (3) Eastern Flank of the Ghaba and South Oman Salt Basins: characterised by shallow Huqf Group, Palaeozoic sediments less than 2000 m thick, and thin or absent Mesozoic sediments. The salt dissolution structures that are typical for the southern part of the east flank are not present in the northern part of Area 3. (4) Tertiary Basins: Huqf Group partly eroded or absent, Palaeozoic absent over most of the area, and thick Tertiary sediments containing (deep) marine sediments. (5) Mountain Front: foredeep of the Oman mountains, with very thick Campanian clastics. In this paper we will concentrate on Areas 1, 2, and 3, because these are the most important for the generation of Huqf oil and gas. Area 5 is the main kitchen area for Middle Cretaceous Natih derived hydrocarbons, and Area 4 is still very poorly known. Methods

Fig. 4. Locations of 97 wells used in maturity calculations; hydrocarbon generation domains; calibration data. A, B, C, D, E, F, G, K, and L: wells referred to in the text. X= maturity data Huqf source rocks (based on solid hydrocarbon reflectivity); Y=vitrinite data Permian Haushi coals; Z=fission track data. Hydrocarbon generation domains: l = R u b ' a l Khali Flank; 2=South Oman and Ghaba Salt Basins; 3 = Eastern Flank; 4 = Tertiary basin; 5 = Mountain Front.

five areas with distinct hydrocarbon generation histories (Figs. 1 and 4). These are: ( 1 ) Rub' al Khali Flank: covering the area to the west of the Ghaba and South Oman Salt Basins, and comprising the Fahud Salt Basin in the north. (2) Ghaba and South Oman Salts Basins: covering the deepest axial part of the study area, where massive salt and Lower Haima sand are present. In the north, the basin is almost symetrical, with a steeper eastern flank due to Devonian uplift. In the south, the Cambrian transpression modified the west flank ge-

The hydrocarbon generation history was modelled with an in-house geohistory modelling package, which solves the one-dimensional heat diffusion equation. The program uses as input mantle heat-flow history, heat capacity and thermal conductivity for the various lithologies, surface temperature history, and burial history. Calculations are iterated by adjustment of the crustal radiation constant, until the calculated present-day temperature-depth profile fits the data, which arexderived from well log measurements. Full decompaction is applied in the reconstruction. Heat transfer by convection is not taken into account. The main input parameters are given below.

Temperature data and surface heat flow Literature data on heat flow and geothermal gradients for the Sultanate of Oman are restricted to a heat flow value in the Gulf of

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Oman (42 m W / m 2, Hutchison et al., 1981) and some temperature measurements in hot springs in the Oman Mountains. For this study temperature data from 97 exploration wells were used to create a present-day surface heat flow map for the interior of Oman. From a data set of 165 wells, the best 97 were selected based on the number of logging levels, number of logging runs per level, quality of the measurements, total depth of the well, and geographical distribution of the data set. Logging temperatures were corrected for the effect of drilling mud circulation to obtain virgin formation temperatures, following a method similar to the CTRM procedure (Dawdle and Cobb, 1975; Roux et al., 1980). In the calculated temperature profile (Fig. 5 ) the general curvature is due to increasing compaction with depth. The effect of different lithologies and densities, especially salt, on the geothermal gradient is clearly illustrated. The corrected temperature data were then used to derive the present-day surface heat flow (Fig.

The palaeo heat flow (Fig. 7) was assumed to be dominated by two factors: firstly, a general decay of heat flow with time, due to the decrease in concentration of radioactive elements with time; secondly, a heat pulse associated with the Devonian uplift. The magnitude of this heat flow anomaly was estimated according to the model of Mareschal ( 1983 ), which relates magnitude of uplift to the magnitude of heat flow anomaly. It is realised that a number of other heat flow pulses might have occurred. For example, the rifting event during the late Huqf, which created the Ara Salt Basins, must have been accompanied by a significant heat flow pulse. However, the Middle Huqf source rocks were buried to less than 2000 m at that time, and will not have reached the oil generation window. Also the tectonic events related to the Triassic break-up unconformity must have been accompanied by a heat pulse. However, fission track analysis (see below) shows that none of the Mesozoic heat pulses led to elevation of subsurface temperatures beyond the temperatures reached by the end Haima, and therefore are of little importance to the hydrocarbon generation history. The minor heat flow anomalies are very difficult to quantify at present, and thus it was decided to apply the rather simple heat flow curve of Fig. 7.

Surface temperature history The variation in surface or water/sediment interface temperatures through geological time has a significant effect on maturity calculations. The present-day annual average surface temperature is about 25 ° C, measured at 10 cm

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depth in the soil of the central desert. The palaeo surface and water/sediment interface temperatures were estimated from the palaeo latitude of Oman, as given by Smith et al. ( 1981 ). In addition, lithology was taken into account: e.g. terrestrial versus shallow versus deep marine, and tillites as opposed to evaporites. The result (Fig. 7) shows as much as 25 °C variation through time. As the accuracy of the palaeo-latitude estimates decrease rapidly with geologic age, lithology was an important criteria to assess the palaeo surface tem-

perature for the Huqf Group. The far southerly or northerly latitude during the Early Huqf is based only on the occurrence of tillites in the Lower Huqf Group Abu Mahara Formation, and the very high temperature during the late Huqf is based on water temperatures in shallow evaporating basins in arid climates.

Burial history The statigraphy (Fig. 2) shows some eight major regional unconformities. The overbur-

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almost the entire Silurian, Devonian, and Carboniferous missing (Figs. 2 and 9). The Devonian lacustrine Misfar Formation is preserved only in a few local depressions in the South Oman Salt Basin (Hughes Clarke, 1988). The unconformity at the base of the Haushi Group cuts into increasingly older formations towards the east: subcrops range from the Silurian Haima Group in Area 1, to the Ordovician Haima Group in Area 2, to the Cambrian Haima Group or even Proterozoic H u q f Group in Area 3. In the H u q f Axis outcrop area, glacial striations made by Haushi Group boulders into H u q f carbonates are very well exposed (Braakman et al., 1982; Hughes Clarke, 1988 ). Thinning of the individual formations within the Ordovician/Silurian Haima Group is not very pronounced, while the Cambrian Lower Haima Group forms a clear easterly thinning wedge. From this re-

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gional evidence it can be concluded that missing overburden at the base of the Haushi Group unconformity increases eastward, and that at least part of the present westerly dip of the Huqf monocline is due to differential uplifting during the Devonian. Quantification of this missing overburden, however, is very complex and involves a number of different lines of reasoning as discussed below: Compaction of the Haima and Haushi soft shale layers was used to provide an estimate of the missing overburden. The compaction trend of the soft shale exhibits a clear break at the H a u s h i / H a i m a boundary (Fig. 10), which indicates 1800 m of missing overburden for this particular well. Some 30 wells in south Oman were analysed with this method, which suggest that 1200-2500 m of sediments were eroded

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by the basal Haushi unconformity. The low accuracy of the method, however, does not permit the detection of a clear trend of increasing missing overburden eastward. Maturity data from Huqf source rocks, mainly based on solid hydrocarbon reflection measurements (Jacob, 1989), show similar VRE ( =Vitrinite Reflectance Equivalent)

PROTEROZOIC

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25

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values in well K at 1200 m depth (Fig. 11C), as in well C at 2800 m, or at 4000 m depth in well F (Fig. l i D ) . Assuming the same heat flow history, and similar lithologies in the (now missing) overburden, these data suggest 10002500 m more erosion and uplift in the area of well K as compared with wells C, E or F. Fission track analysis of apatites (Gleadow et al., 1983, 1986; Green et al., 1989; Laslett et al., 1987) were carried out on 84 Haushi, Haima, and H u q f samples from eleven wells Vltrinite Reflectance (%)

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mostly situated in Area 3 (Fig. 4). The detrital apatites in the H u q f and Haima Groups are derived from granitic basement that is dated 650-750 Ma (Hughes Clarke, 1988 ). The apatites in the Haushi Group are either from reworked Haima Group or from the basement. The corrected fission track ages of these apatites cluster around 400 Ma, indicating a considerable uplift around that time (Fig. 12). Samples with low ( < 350 Ma) corrected fission track ages are all from deep present-day burial where the tracks have been partially annealed. The fission track length distribution patterns suggest that none of the Mesozoic unconformities represents a significant amount of missing overburden, and that in the Oligocene the sediments were at a temperature of 1030 °C above the present. The wells C, D, E, F, G, and K (Fig. 4 ) were used to calibrate the missing overburden at the basal Haushi unconformity. These wells have

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26

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Fig. 13. Typical burial graphs for Areas 1, 2 and 3 (for location of wells see Fig. 4). For key to Nos. 1-7 see Figs. 5 and l I. Heavy lines are Upper and Middle H u q f source rock levels used in the calculations. Dashed lines are iso-VRE curves for the base and top oil generation window (0.62 and 1.2) and top of the main gas generation window (2.4). Total depth for well A = 4200 m, for well B = 3300 m, well C = 3300 m. Deeper stratigraphy derived from seismic data.

27

PROTEROZOIC SOURCE ROCKS IN OMAN

relatively good maturity data in the range 0.651.0 VRE, mainly based on solid hydrocarbon reflection measurements. They are situated at different positions relative to the current depth of the Huqf, and therefore would represent different amounts of missing Haima Group sediments. (It should be noted that no wells in more westward locations can be used for calibration, because the Huqf is beyond drillable depth. ) With the assumption of the heat flow curve of Fig. 7, and missing overburden at base Sahtan to be less than 600 m, the amount of missing Haima was varied until the calculated maturity matched the observed values in the 6 calibration wells (Fig. 11 ). Subsequently, the contours of equal missing overburden at the base of the Haushi Group were drawn roughly parallel to the depth contours of the Huqf (Fig. 9). The contours are heavily smoothed because of the poor accuracy of the data. The map suggests a maximum missing overburden over the present-day offshore region, i.e., where the Indian-Arabian break-up occurred. Construction of burial graphs for the 97 wells was based on the well information, depositional/erosional isopach maps, and seismic data. The latter was used to extend the well data down to the Huqf Group source rock level in those localities where the Huqf was not penetrated by wells. Typical examples of burial graphs for each of the three main hydrocarbon generation domains are shown in Fig. 13. In Area 1 (Well A, Fig. 13 ) the Huqf was buried to about 3000 m by Early Silurian sediments. A long period of mild uplift and erosion/non deposition followed during the late Palaeozoic. Almost continuous subsidence, interrupted by short periods of erosion/non-deposition characterises the Mesozoic and Tertiary, during which some 4000 m of mainly shallow-water carbonates were deposited, burying the Middle Huqf source rocks to about 67 km depth and the Upper Huqf source rocks to about 5 km depth. Through all this time the basin subsidence was very slow. The average sedimentation rate is about 14 m/Ma, and even

during the Akhdar period with relatively fast subsidence, the sedimentation rate does not exceed 60 m/Ma. In Area 2 (Well B, Fig. 13) the HuqfGroup was buried to great depth (up to 8000 m) by the Early Silurian, followed by erosion of less than 1500 m during the Devonian. The Mesozoic and Tertiary subsidence was slow and had some periods of limited uplift. In detail the amounts of uplift and erosion changes from north to south within Area 2, but these differences are of subordinate importance for the hydrocarbon generation history. Area 3 (Well C, Fig. 13 ) is very well documented in the south, but poorly known in the north, where only a few wells have been drilled. Burial of the Huqf was less deep by the Early Silurian than in Area 2. Uplift in the Devonian on the other hand was far more pronounced. Subsidence in the Mesozoic was restricted to a few hundreds of meters, and repeated periods of uplift caused erosion of previously deposited Mesozoic formations. Only during the early Tertiary did significant subsidence resume. Area 4 is similar in its burial history to Area 3, but the uplift and erosion during the Devonian is more severe, and can amount to several thousands of meters. In the offshore wells the HuqfGroup is partly or completely eroded, and the Mesozoic directly overlies the Proterozoic strata. In Area 5 the Huqf Group was buried to great depths before the Devonian. The area is not further considered in this paper because it has no great significance for the Huqf derived oil and gas occurrences. Discussion

The surface heat flow map of Oman (Fig. 6) shows a very clear relationship with the tectonic feature map (Fig. 1 ). The very low heat flow values roughly coincide with the centre of the salt-filled Upper Huqf graben. In between the Ghaba and South Oman Salt Basins an area

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PROTEROZOIC SOURCE ROCKS IN OMAN

of slightly more elevated surface heat flow exists. The H u q f in this area rises to a generally shallower level. This not is du~ to a late Palaeozoic or younger tectonic event, but is thought to have been created during the Upper H u q f rifting event. Well L has an exceptionally high heat flow (58 m W / m 2 ) , which could be due to the fact that the well was drilled into a salt dome at about 2200 m depth. The well directly to the east (heat flow 48 m W / m 2) on the other hand, was drilled into a Haima turtle-back structure between salt walls. The two wells west of L have a surface heat flow of 64 and 59 m W / m 2, respectively, far higher than any value in the area. These two wells are situated in a heavily tectonised, poorly understood area. The surface heat flow is somewhat higher towards the east in Area 4, where the upper Palaeozoic or Mesozoic sediments directly overlie the H u q f Group. This area is thought to have been subjected to large-scale transtensional movements during the Triassic/Jurassic, and to transpressional movements during the Late Cretaceous. Also westward in the subsidence domain of the Rub' al Khali Basin surface heat flow is higher than in the axial part of the basin. Here the Akhdar carbonates as well as the Mesozoic section are far thicker than in the east, while the Haima is relatively thin, and the H u q f salt is missing. Tectonically the area has been a slow gently subsiding basin since the Permian, without major tectonic events. The relatively large fluctuations in heat flow over short distances might be due to convection of subsurface waters: hot deep fluids might flow to shallower levels via faults, or meteoric waters might be introduced into deeper levels via convection cells (Luheshi and Jackson, 1986; Goblet et al., 1986 ). The density of data points does not permit evaluation of these effects in detail. The very low ( < 40 m W / m 2) values in the axial part of the Proterozoic basin are low even compared to Archean and Proterozoic shields, which have an average heat flow

29

of 40-50 m W / m 2 (Jessop et al., 1984). Considering the general age of the study area (740870 Ma, Hughes Clarke, 1988) and the fact that the last main rifting event occurred at 600 Ma, it seems possible that the present-day surface heat flow is affected by processes other than conductive heat flow alone. The possibility cannot be excluded that in the massive package of Haima sands some element of convective heat flow exists. Aquifers in the Haushi and Haima Groups could in principle be charged by meteoric waters along the Huqf Axis, introducing cool fluids into Haima Group convection cells. This process may have been operative during various geological periods after the westerly tilt of the basin was established in the Devonian. However, the effects of this process on the maturation history of the H u q f source rocks cannot be quantified at present. Even with these uncertainties, it is thought that relative to the heat flow history data, the burial history has even larger uncertainties, as explained in the previous section. The maturity calculations that are based on these basic input data will therefore only provide a global impression of the generation history, and no great precision can be claimed. Figures 14 and 15 present four typical temperature, VRE, and generation history curves for the H u q f source rocks for each of the different areas. In Area l the temperature generally increases with time, except during some periods of cooling associated with the major uplifts, and the m a x i m u m temperature of 160-190 ° C is reached just before present. In Area 2 (Well B, Fig. 15 ) the temperature history is very different: the temperature rapidly increases to values as high as 220°C by the end Haima times, and thereafter it stays below that level, with minor fluctuations related to the periods of uplift or subsidence. Area 3 (Well C, Fig. 15) again shows a fundamentally different temperature profile: the m a x i m u m burial temperature of about 120 ° C is reached at the end

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PROTEROZOIC SOURCE ROCKS IN OMAN

Haima, and is significantly higher ( > 3 0 ° C ) than at any time thereafter. It should be emphasised that this result is brought about by the choice of heat flow history (Fig. 7), and estimate of missing Haima overburden, which was based on calibration by VRE data (Fig. 11 ). The calculated subsurface temperature history is in agreement with the data from the fission track analysis, which showed that the maximum burial temperature was reached around 400 Ma. The calculated VRE history curves for the H u q f i n Area 1 (Fig. 14) show a stepwise increase through time. The Middle Huqf source rocks become post-mature for oil by the end Jurassic, and the Upper Huqf source rocks enter the gas phase by the end Cretaceous. In Areas 2 and 3 the calculated VRE rapidly increases until the end Haima, and thereafter remains at about the same level (Fig. 15 ), the main difference between the two areas being the maturity level reached: post mature for oil and gas in Area 2 and mature for oil in Area'3. Generation of hydrocarbons only happens when the maturity level of the source rock increases. This is particularly well displayed in the example from Area 1 (Fig. 14 ), where generation from the Middle Huqf starts in the Late Ordovician, stops in the Early Devonian, no generation occurs during the Devonian-Carboniferous uplift period, and generation resumes in the Late Permian-Early Triassic when the basin is subsiding again. The Upper Huqf shows an even more complex generation pattern, which can be directly correlated to the rate of VRE increase. The generation curves for Areas 2 and 3 (Fig. 15 ) illustrate that few hydrocarbons have been generated during the last 400 Ma. The present-day maturity map of the Middle Huqf is based on 97 wells (Fig. 16). It shows the source rocks to be mature for oil in Areas 3 and 4, but post mature for oil and gas over most of Areas 1 and 2. The maturity map for the Upper Huqf source rocks (not presented here) shows a very similar pattern, but

31

with slightly lower values. The Upper Huqf source rocks are presently only just mature for oil in Area 3, and are in the gas window over large parts of Areas 1 and 2. The Upper Huqf source rock layer is assumed to directly overlie the Ara salt, which is intensively structured by halokinetic movements that took place mainly during the Cambrian, with some reactivation phases during the Mesozoic and Tertiary in north Oman. The maturity of the Upper Huqf source rocks, therefore, can be significantly different between a position on a salt high or in a valley between two salt walls. From the discussion of the generation history curves it will be evident that the presentday Huqf kitchen maps reflect a palaeo-kitchen for most of interior Oman. The fact that at present Area 3 is mature for oil, does not necessarily imply that oil is actually generated at present. This is illustrated by "time of oil generation maps" (Fig. 17) and by "cumulative generation curves" (Fig. 18 ). The Middle Huqf stopped generating oil 400 Ma ago over most of Oman, with the exception of the Fahud Salt Basin in Area 1. The Upper Huqfgenerated oil before the Silurian in Areas 2 and 3, but in Area 1 the main phase of generation is the Mesozoic, in the Fahud Salt Basin even as late as the Upper Cretaceous. The immature values calculated for parts of Area 2, are caused by a structural high between the Ghaba and South Oman Salt Basins, and by effects of halokinetic structuration (Well L). The cumulative hydrocarbon generation curves for Areas l, 2 and 3 (Fig. 18 ) illustrate the main problem in the hydrocarbon habitat of Oman: In Area 1 most oil reserves have been found in traps sealed by Cretaceous shales of the Wasia or Aruma Formations. So far, only limited reserves have been found in Akhdar Group traps. Considering the fact that these oils are highly mature (Grantham et al., 1988) they were probably generated during the Jurassic or Cretaceous (Fig. 14). If the oils were generated from the Upper Huqf during the Late

32

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Cretaceous, direct migration into existing traps was possible. If on the other hand the oils originate from the Middle Huqf, the main generation phase was during the Late Permian-Early Jurassic, and a remigration model would have to be invoked to explain Upper Cretaceous oil pools. At present, geochemical data do not permit the discrimination between Upper or Middle Huqfprovenance (Grantham et al., 1988 ). In Areas 2 and 3 almost all reserves are found in Haima and Haushi reservoirs sealed by

Haushi, Akhdar, or Wasia shales. Therefore all hydrocarbons were generated before any of the traps in which hydrocarbon reserves have been found were formed (Fig. 18). Consequently, hydrocarbon habitat models have to invoke a scheme of intermediate storage and remigration (Koonert and Visser, 1991 ). It should be noted here that some of the oil fields, mainly in Area 1, are filled with Silurian, Jurassic, or Cretaceous oils (Grantham et al., 1988). Although this paper is not con-

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MILLIONS OF YEARS Fig. 18. Average cumulative generation curves for Middle Huqf (heavy lines) and Upper Huqf (thin lines) source rocks. Dots-dashes: Area 1, dashes: Area 2, solid lines: Area 3. Main phases of trap formation: /=deposition Ara salt; 2=deposition Lower Haima shales; 3=halokinesis; 4=deposition Haushi shales and diamictites; 5=deposition Khuff red beds; 6 = deposition middle Cretaceous shales; 7 = upper Cretaceous structuring and shale deposition; 8 = structuring by salt dissolution.

cerned with the generation history of the younger source rocks, it will be evident from the above, that most of these younger oils were generated after trap formation. The habitat of the "Q" oils is not discussed here, because their source is unknown, except for the notion that these oils probably were generated from a Proterozoic source rock (Grantham et al., 1988 ). The results of this basin evaluation may contribute to the understanding of the mechanism of the opening of the Indian Ocean in the lower Mesozoic. The following data suggest that the Huqf Axis (Fig. 1 ) has been a high through most of its Phanerozoic history: Seismic profiles show that the Middle Huqf truncates the Lower Huqf along an angular unconformity in Area 3, while evidence for such a relationship is missing in Area 2. - The Middle Huqfgenerally thins from Area 2 toward Areas 3 and 4, which is mainly due to erosion at its top during the formation of the Upper Huqf graben (V. Carayon, pers. commun., 1988). - The Upper Huqf thins eastward. -

- The Lower Haima forms a clear westward thickening wedge, while sedimentary thickness variations within the Upper Haima are not evident. - The Haushi thickens westward, and the sediments fine in that direction. - The Akhdar shows some 50% thinning from the border with Saudi Arabia to the eastern flank of the South Oman and Ghaba Salt Basins. Also the sediments change in facies from shallow marine carbonates in the northwest to terrestrial fine red-beds in the southeast. A number of the Jurassic and Cretaceous formations present in Area 1 are missing in Area 3. However, some of the carbonate formations of the Upper Jurassic and the Cretaceous have been encountered in offshore wells, and they are very similar to their age equivalents in Area 1. - The Tertiary is the only succession that is gently thickening from west to east in southern Oman. In north Oman it is entirely eroded in the eastern area. From evidence it is clear that the eastern -

35

PROTEROZOIC SOURCE ROCKS IN OMAN

margin of Oman has remained a high over a very long period of time. Husseini (1988) has shown that the eastern part of Oman must have been located between two latest Proterozoic grabens, which evolved into the Upper Huqf salt basin in Oman and the Salt Range Salt Basin in Pakistan. Areas 3 and 4 therefore are thought to have been part of an extensive high region between these two grabens. This high was subsiding during the Ordovician and Silurian, but formed the centre of uplift during the Devonian, when an uplift of several kilometers took place. The glacial deposits from the Haushi Group provide evidence for a mountain range to the east at that time (Braakman at al., 1982 ), and also the facies change in the Akhdar Group from carbonates to clastics point to a source area to the southeast. Following the break-up in the early Mesozoic, massive thermal subsidence, at least similar in magnitude to the preceding uplift, could be expected to have occurred, assuming the developing margin to have been of the classical Atlantic passive margin type. As we have seen this is not the case and instead of 2000-4000 m of Mesozoic section, only a few hundreds of meters were deposited. Fission track data exclude the possibility of more than 1500 m Mesozoic sediments in Areas 3 or 4. From this evidence we conclude that the break-up between India and Arabia cannot have followed a classical rift-drift pattern. Instead the data from the interior wells support a breakup mechanism which invokes a transform fault (Besse and Courtillot, 1988; Barton et al., 1990).

northwestern part of the country was oil generated from the Huqf during the Mesozoic. These results have important consequences for the hydrocarbon habitat models of Oman, because most of the traps were not in place prior to hydrocarbon generation. Models involving remigration will have to be developed to explain the occurrence of most oil fields in south Oman. The" geological data accumulated for this basin study show that the eastern margin of Arabia remained a high for a very large part of its geological history. From this it is concluded that the onshore data support the concept that the breakup between India and Arabia took place along a transform fault.

Conclusions

Aley, A.A. and Nash, D.F., 1984. A summary of the geology and oil habitat of the Eastern Flank Hydrocarbon Province of South Oman. Proc. OAPEC Seminar on the Source and Habitat of Petroleum in the Arab Countries, Kuwait, October t 984. Barton, P.J., Owen, T.R.E. and White, R.S., 1990. The deep structure of the east Oman continental margin: preliminary results and interpretation. Tectonophysics, 173: 319-331. Besse, J. and Courtillot, V., 1988. Paleogeographic maps of the continents bordering the Indian Ocean, since the early Jurassic. J. Geophys. Res., 93:11,791-11,808.

The Proterozoic sedimentary basin in the interior of Oman has generated hydrocarbons from Huqf source rocks of Proterozoic age. Deep burial of these source rocks, followed by strong uplift in the Devonian, little subsidence thereafter, and elevated palaeo heat flow resuited in generation of hydrocarbons before the Devonian over most of the basin. Only in the

Acknowledgements This work was carried out during an assignment with Petroleum Development Oman. Many of my colleages there have contributed to this project. The often heated discussions have been of great benefit for the development of the models presented in this paper. Reviews by J.R. Willcox and an anonymous reviewer have been greatly appreciated. I am indebted to Petroleum Development Oman LLC, the Ministry of Petroleum and Minerals of the Sultanate of Oman, and to the Shell Internationale Petroleum Maatschappij for permission to publish this paper.

References

36 Blendinger, W., 1989. Permian to Jurassic deep water sediments of the eastern Oman Mountains: their significance for the evolution of the Arabian margin of the south Tethys. Facies, 19: 1-32. Braakman, J.H., Levell, B.K., Martin, J.H., Potter, T.L. and van Vliet, A., 1982. Late Palaeozoic Gonwana glaciation in Oman. Nature, 299 (5878): 48-50. Dowdle, W.L. and Cobb, W.M., 1975. Static formation temperatures from well logs---an empirical method. J. Petrol. Technol., 27:1326-1330. Gleadow, A.W.J., Duddy, I.R., Green, P.F. and Lovering, J.F., 1983. Fission track analysis; a new tool for the evaluation of of thermal histories and hydrocarbon potential. APEA J., 23: 93-102. Gleadow, A.W.J., Duddy, I.R., Green, P.F. and Lovering, J.F., 1986. Confined fission track lenghts in apatite-a diagnostic tool for thermal history analysis. Contr. Miner. Petrol., 94:405-415. Glennie, K.W., 1977. Outline of the geology of Oman. Mem. Ser. Geol. France, 8:25-31. Glennie, K.W., Boeuf, M.G.A., Hughes Clarke, M.W., Moody Stuart, M., Pilaar, W.F. and Reinhardt, B.M., 1974. Geology of the Oman Mountains. Verh. Kon. Nederl. Geol. Mijnb. Genoots., 31: 1-423. Goblet, P., Ledoux, E. and Marsily, G., 1986. Possibilities of abnormal heat in sedimentary basins: some examples. In: J. Burrus (Editor), Thermal Modelling in Sedimentary Basins. 1st IFP Exploration Research Conf., Carcans, France. Technip Collection Colloques et Seminaires No. 44, pp. 235-246. Gorin, G.E., Racz, L.G. and Walter, M.R., 1982. Late Proterozoic sediments of the Huqf Group, Sultanate ofOman. AAPG Bull., 66: 2609-2627. Grantham, P.J., Lijmbach, G.W.M., Posthuma, J., Hughes Clarke, M.W. and Willink, R.J., 1988. Origin of crude oils in Oman. J. Petrol. Geol., 11: 61-80. Green, P.F., Duddy, I.R., Gleadow, A.J.W. and Lovering, J.F., 1989. Apatite fission track analysis as a paleotemperature indicator for hydrocarbon exploration. In: N.D. Naeser and T. McCulloh (Editors), Thermal History of Sedimentary Basins-- Methods and Case Histories. Springer, New York, pp. 181 - 195. Hughes Clarke, M.W., 1988. Stratigraphy and rock unit nomenclature in the oil producing area of Interior Oman. J. Petrol. Geol., 11: 5-60. Husseini, M.I., 1988. The Arabian Infracambrian extensional system. Tectonophysics, 148: 93-103. Hutchison, I., Louden, K.E., White, R.S. and von Herzen,

w. VISSER R.P., 1981. Heat flow and age of the Gulf of Oman. Earth Planet. Sci. Lett., 56, 252-262. Jacob, H., 1989. Classification, structure, genesis, and practical importance of natural solid oil bitumen ("Migrabitumen"). Int. J. Coal Geol.: I 1, 65-79. Jessop, A.M., Lewis, T.J., Judge, A.S., Taylor, A.E. and Drury, M.J., 1984. Terrestrial heat flow in Canada. Tectonophysics, 103: 239-262. Kroonert, G., Visser, W. and van den Brink, H., 1991. Generation, migration and entrapment of Precambrian oils in the eastern flank heavy oil province, South Oman (abstr.). Am. Assoc. Petrol. Geol. Bull., 75: 1413. Laslett, G.M., Green, P.F., Duddy, I.R. and Gleadow, A.J.W., 1987. Thermal annealing of fission tracks in apatite 2. A quantitative analysis. Chem. Geol. (Isot. Geosci. Sect. ), 65: 1-13. Lippard, S.J., Shelton, A.W. and Gass, I.G., 1982. Ophiolites of northern Oman. Mem. Geol. Soc. London, 11: 1-178. Luheshi, M.N. and Jackson, D., 1986. Conductive and convective heat transfer in sedimentary basins. In: J. Burrus (Editor), Thermal Modelling in Sedimentary Basins. 1st IFP Exploration Research Conf., Carcans, France. Technip Collection Colloques et Seminaires No. 44, pp. 219-234. Mareschal, J.C., 1983. Mechanisms of uplift preceeding rifting. Tectonophysics, 94:51-67. Moseley, F., 1988. Structure of the Masirah ophiolite, Oman. In: A.H.F. Robertson, A.E.S. Ries and J.D. Smewing (Editors), The Geology and Tectonics of the Oman Region. Univ. of Edingburgh, Edingburgh, p. 36 (Abstr.). Murris, R.J., 1981, Middle East, stratigraphic evolution and oil habitat. Geol. Mijnb., 60: 467-486. Roux, B., Sanyal, S.K. and Brown, S.L., 1980. Improved method for estimating true reservoir temperatures from transient temperature data. Rep. No. 80888, Soc. Petrol. Eng. AIME. Shackleton, R.M. and Ries, A.C., 1988. Structural evolution ofNE Oman and Masirah Island. In: A.H.F. Robertson, A.E.S. Ries and J.D. Smewing (Editors), The Geology and Tectonics of the Oman Region. Univ. of Edingburgh, Edingburgh, p. 52 (Abbr.). Smith, A.G., Hurley, A.M. and Briden, J.C., 1981. Phanerozoic Paleocontinental World Maps. Cambridge University Press, Cambridge.