Water consumption in hydrocarbon generation and its significance to reservoir formation

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 2, April 2013 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(2): 259–267.

RESEARCH PAPER

Water consumption in hydrocarbon generation and its significance to reservoir formation WANG Yongshi*, ZHANG Shouchun, ZHU Rifang Geoscience Research Institute, Shengli Oilfield Company, Sinopec, Dongying 257015, China

Abstract: The geochemical effects of water consumption during hydrocarbon generation were studied on the basis of evolution laws of source rocks and simulation experiments on hydrocarbon generation. Water consumption statistics were obtained in order to study the relationship between water consumption during hydrocarbon generation and hydrocarbon migration and reservoir formation. The simulation experiments of hydrocarbon generation were performed under hydrous and anhydrous conditions for correlation. The geochemical characteristics of organic evolution under these two conditions were analyzed and the variations of hydrocarbon generation potential and carbon transformation ratio were emphasized. The results show the effects that organic matter and water have on each other during hydrocarbon generation: part of unavailable carbon is activated in kerogen and hydrogen is increased in degraded products, which leads to the increase of total hydrocarbon generation potential. According to water consumption mechanisms, the quantitative evaluation method of water consumption in hydrocarbon generation was put forward and used in the studies of the main source rocks in the Dongying Sag. Both of the water consumption and the depth range of the Upper Es4 Member are larger, while those of the Lower and Middle Es3 Members are smaller. Water consumption affects hydrocarbon migration and accumulation by increasing organic carbon degradation rate to increase fluid volume. Pore fluid pressure and oil-bearing saturation are consequently increased. The matching relationship between water-consuming hydrocarbon generation intervals and water-consuming diagenesis intervals enhances the dynamic forces of hydrocarbon migration, which benefits the formation of self-generating and self-preserving reservoirs or lower-generating and upper-preserving reservoirs. Key words: hydrocarbon generation; water consumption; reservoir formation; simulation; Dongying Sag

Introduction Widely occurring underground in sedimentary rocks, formation water is not only the carrier of fluid potential that controls the dynamics of petroleum migration, but also plays a role in diagenesis as the reaction medium for a variety of reactions. Research shows that "water consumption" caused by mineral alteration during diagenesis, leads to substantial reduction in formation water, leaving reservoirs in a low-pressure state [1]. In source rocks, hydrocarbon generation is an important factor as regards changes in fluid properties. It is not only an organic reaction, but also exerts influence on inorganic minerals and formation water. The catalytic effect of inorganic minerals on hydrocarbon generation has been commonly acknowledged. In contrast, sparse attention has been paid to the effect of water on hydrocarbon generation. Current understanding in this regard can be summarized as follows [2-3]: (1) The presence of water suppresses hydrocarbon generation somewhat, inasmuch as water itself has the ability to raise pressure; furthermore, kerogen can through exchange or by

bonding water molecules into its structure, become rich in hydrogen; (2) The combination of water and organic matter mainly occurs in the middle diagenetic stage, where Ro values are roughly equivalent to 0.3% – 0.7%; (3) Water can be dissolved in generated bitumen, which, in turn, promotes the process of hydrogenation, i.e. as a secondary reaction. In short, the existence of water influences the composition and quantity of hydrocarbon products. This paper takes a close look at water-consumption mechanisms and the extent thereof during hydrocarbon generation, with a view to revealing the relationships between water consumption and oil-gas migration and accumulation.

1

Samples and methods

Taking source rocks occurring in the Dongying Sag, Shengli Oilfield, as study object (representing as such the Palaeogene source rocks in continental rift basins), the study analyzed the geochemical effects of water consumption during hydrocarbon generation, as well as the mechanisms thereof, through a combination of natural-evolution dissection and

Received date: 13 Oct. 2012; Revised date: 25 Jan. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Sinopec Science and Technology Major Project (P07009). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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Table 1

Geochemical parameters of the simulation samples

Stratigraphic position

Lithology

TOC/%

Ro/%

Chloroform extract “A”/%

S1/ (mg·g−1)

S2/ (mg·g−1)

Experimental condition

B1

Lower Es3

Grey-brown oil shale

9.03

0.31

0.481 8

0.98

57.45

30 MPa hydraulic pressure

B2

Lower Es3

Grey-brown oil shale

10.98

0.31

0.690 4

1.56

70.38

30 MPa plunger-exerted pressure

Experimental number

artificial simulation. Based on occurrence in the source rocks, the total water consumption was estimated. The Dongying Sag displays three sets of source rocks, formed in different depositional environments. They are respectively the Upper Es4, the Lower Es3 and the Middle Es3 Members. The Lower Es3, from which samples were taken for artificial simulation (Well Bin 338-6), lies midway in the sequences. The geochemical characteristics of the samples used for simulation, are shown in Table 1. The organic-carbon content clearly reflects the evolutionary process. For purposes of determining the influence of minerals during hydrocarbon generation and in order to facilitate analysis of the original hydrocarbon-generation potential, total rock was pulverized prior to simulation. The simulation was conducted in a device with two types of autoclave, capable of meeting different experimental conditions, and referred to as the “Experimental simulator of oil-gas generation and migration”. The autoclaves are cylindrical with straight-through cavities which can be sealed with flange covers and in which stainless steel blocks can be put at both ends for the adjustment of sample position. The difference between the autoclaves is that one is installed with a plunger for imposing vertical stress, while the other, without a plunger, imposes pressure by injecting fluid. The imposed pressures are all controllable. The samples were pulverized into particles of 0.18 mm in diameter. The experiments were carried out under two distinct experimental circumstances, namely simulation under hydraulic pressure (B1) and simulation under plunger-exerted pressure (B2). Because a large amount of samples would be required for the experiments, the samples for the two circumstances were taken on two occasions. The sampling locations were practically identical so as to maintain consistency in the characteristics of the samples. In the simulation using water, 6 temperature points, namely 200 °C, 250 °C, 275 °C, 300 °C, 325 °C and 350 °C were designed, with the samples placed in the middle of the autoclave and sealed, and distilled water being injected into the reactor (water and samples in full contact) by means of a constant-pressure pump. After having been filled with water, the hydraulic pressure was maintained at 30 MPa. For purposes of comparison, the simulation using the plunger was conducted in the main hydrocarbon-generation stage at the following temperature points: 275 °C, 300 °C, 325 °C and 350 °C. As regards stress factors, vertical pressure of 30 MPa was applied by pressing the plunger. The experiment was designed stepwise as follows: At first, apply pressure up to the target value, and then raise the temperature while maintaining the pressure. After reaching the designed

temperature, the conditions were maintained for 48 hours.That completed the main process of the experiment. The temperature was then decreased to about 60 °C and the mixing fluid was removed and separated. Gaseous products were collected and measured, using the drainage method. Expelled oil was extracted with dichloromethane and measured by applying the constant weight method. Residual rock samples were taken out and dried for essential geochemical tests. The experimental conditions were designed to reflect the effect of formation water at different evolutionary stages in source-rock evolution. Gas chromatography was used for testing gaseous products for C1-C5 and CO2 content. The residual rock samples were tested, using the following methods: Residual liquid hydrocarbons were extracted by chloroform, for 8 hours (chloroform extract “A”); hydrocarbon-generation potential parameters (S1 representing volatile hydrocarbon at 300 °C, S2 representing thermal cracking hydrocarbons at 300–600 °C) were determined by Rock-Eval pyrolysis; after inorganic carbon was removed by dilute hydrochloric acid, the total whole-rock organic-carbon content (TOC) was measured by the combustion method; Kerogens in the samples were obtained for vitrinite reflectance (Ro) testing, after alkali treatment and heavy-liquid separation.

2 Geochemical effects of water consumption in hydrocarbon generation 2.1 Hydrocarbon-generation potential in natural evolution A comparison of the products from the two types of simulation shows that the products of simulation with water more closely reflect the geological evolution [4-6], underlining the importance of water to hydrocarbon generation. The hydrocarbon generation-potential index (GI) is an effective indicator to denote hydrocarbon-generation capacity, its quantitative expression being as follows: S + S2 GI = 1 × 100% (1) TOC Formula (1) shows that where hydrocarbons are in different states of transformation, GI will, in the absence of added or expelled hydrocarbons, remain constant. GI may decrease with the partial expulsion of hydrocarbons. Before the hydrocarbon-expulsion threshold is reached, GI represents the original hydrocarbon-generation capacity. After entering the threshold, GI represents the remaining hydrocarbon generation-capacity. In the 3 sets of source rocks from the Dongying Sag, the Upper Es4 and the Lower Es3 Members contain typical Type Ⅰorganic matter, while the Middle Es3 Member, being com-

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plex in organic matter, in the main contains Type II organic matter [7]. Changes in the hydrocarbon generation-potential index with depth, as well as in the organic matter-type index (being the weighted sum of organic-matter percentages [8], where, in general, the higher the type index, the stronger the hydrocarbon-generation ability), are illustrated in Figure 1. All GI trends derived from the source rocks, display an initial increase and a subsequent decrease. For the Upper Es4 source rocks, the GI value is about 600 mg/g at a depth of 1 500 m, and reaches a maximum value of about 800 mg/g at 2 500 m. For the Lower Es3 source rocks, the GI value averages roughly around 500 mg/g at 1 500 m, and rises to a maximum of about 600 mg/g at 2 800–3 000 m. The Middle Es3 source rocks, being different in organic-matter types, do not display an obvious GI trend, although a peak value can be discerned at about 3000 m, which then decreases as depth increases. The organic matter-type indices of the Upper Es4 and Lower Es3 source rocks, which remain Type I unchanged, are above 80, while that of the Middle Es3 Member displays wide variation. It is evident that the changes in GI are not entirely caused by organic-matter types. Previous studies show that due to divergence in depositional environments, the hydrocarbon-generation and expulsion thresholds in the source rocks from the study area are quite different [9]. The hydrocarbon-generation threshold of

the Upper Es4 Member is shallower than 2 500 m (Ro<0.4%), and its hydrocarbon-expulsion threshold occurs at about 2500 m. The hydrocarbon-generation thresholds of the Lower Es3 and the Middle Es3 Members are deeper than 2 800 m and 2 900 m (Ro >0.5%), respectively. Their hydrocarbon-expulsion thresholds both occur at about 3 000 m. On natural evolution profiles, the GI increases after reaching the hydrocarbon-generation threshold and then (as residual GI) decreases after reaching the hydrocarbon-expulsion threshold, due to the partial expulsion of hydrocarbons. 2.2 Variation of hydrocarbon-generation potential in simulated evolution

The simulation results of the source-rock sample from Well Bin 338-6 are shown in Table 2 and Fig. 2. In the simulation utilizing water-exerted pressure, Ro values increased significantly (more than 0.4%) as the temperature increased up to 275 °C, at which the organic matter began to enter the mature stage. Correspondingly, the residual liquid products (chloroform extract “A”) increased a great deal, accompanied by expulsion of the oil. The amount of total liquid products reached a maximum at 325 °C, and then decreased at 350 °C. In the simulation with plunger-applied pressure, the Ro value was also greater than 0.4% at 275 °C, while the residual liquid products increased with the rise in temperature. The total liq-

Fig. 1 Relationships between burial-depth, the hydrocarbon generation-potential index and the organic matter-type index of the main source rocks from the Dongying Sag

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Table 2 Temperature/°C 275

300

325

350

Test results of simulated products

Experimental number

Chloroform extract “A”/%

Expelled oil/%

Ro/%

S1/(mg·g−1)

S2/(mg·g−1)

TOC/%

Gas yield/

B1

1.503 6

0.017 6

0.36

2.70

57.80

8.81

7.13

B2

1.199 0

0.059 0

0.43

2.59

69.75

10.96

0.78

B1

2.480 8

0.028 2

0.58

5.01

55.88

8.54

10.47

B2

1.665 7

0.048 8

0.44

2.49

67.09

10.73

2.05

B1

6.681 2

0.201 4

0.76

12.52

54.13

8.05

14.19

B2

4.472 1

0.088 1

0.45

6.11

63.52

10.58

5.71

B1

5.893 4

0.412 0

0.85

9.01

33.04

7.22

26.56

B2

8.513 4

0.279 9

0.61

12.02

57.39

9.95

14.46

(mg·g−1)*

Note: In order to facilitate the evaluation of carbon-element loss, the gas yield is taken as the sum of C1 - C5 hydrocarbons and CO2.

Fig. 2

Relationship between simulated hydrocarbon yields and temperature

uid products reached their maximum at 350 °C. Less liquid oil was expelled with hydrocarbon generation. The produced gases contained both hydrocarbons and CO2. In most cases, liquid-hydrocarbon yields in simulation with water-applied pressure are higher than those in simulation with plungerapplied pressure, the converse only applying at the respective peak values, due to the differences in sample properties and the influence of experimental conditions. Compared with simulation utilizing water-applied pressure, simulation with plunger-applied pressure displays a slow increase in values, a lower rate of degradation in organic matter, less gases generated and less hydrocarbons expelled, which suggests that hydrocarbon-generation capacity under plunger-applied pressure is less than that under water-applied pressure. Compared with the original samples, the GI in simulation with water-applied pressure, only commenced increasing at temperatures ranging from 275 °C to 325 °C (Fig. 3 ), and then went down at 350 °C. The stage during which the GI increases, is consistent with the hydrocarbon-generation period, while the decreasing stage is consistent with the hydrocarbon-expulsion period, once the peak of hydrocarbon generation had been reached. This is similar to natural evolution. The TOC content gradually decreased due to organic degrada-

tion and hydrocarbon expulsion. The invalid carbon content [TOC-0.083 (S1+S2)] shows an exact trade-off with total hydrocarbon-generation potential, with an all-time low at 325 °C. In simulation with plunger-applied pressure, TOC content also decreased gradually with organic degradation and hydrocarbon expulsion, invalid carbon content displayed a weak decreasing trend, while hydrocarbon-generation potential showed only slight change. The comparison between the hydrocarbon-generation potential and the carbon-conversion rate of the two types of simulations shows that added water partially activates invalid carbon to valid carbon in the course of hydrocarbon generation. In addition, the interaction between organic matter and water occurs within a certain temperature range. Previous research results show that a temperature range of 200–350 °C is conducive to the reaction between water and organic matter [9], which is consistent with the results obtained from our experiments. The entire evolutionary pattern indicates that water-added simulation similar to natural evolution, has the capacity to increase hydrocarbon-generation potential, by way of hydrogenation. At present, it is generally believed that the original hydrocarbon-generation potential is constant in recovering calculation. From this study it is evident that that is not the actual situation.

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Fig. 3

Relationship between organic-carbon content and total hydrocarbon-generation potential in simulated evolution

3 Mechanism and calculation of water consumption in hydrocarbon generation 3.1

is as follows:

Water-consumption mechanism

The changes in GI constitute both a comprehensive reflection and effective evidence of water consumption during hydrocarbon generation. Previous geochemical analysis has directly confirmed that water participates in hydrocarbon generation [3,10−16]. It remains, however, difficult to make a quantitative analysis. Hydrogen isotopic changes in hydrocarbons provide obvious evidence of water consumption [13−16]. For example, Hoering conducted experiments using D2O and pre-extracted Messel shale [13] with molecular probes to gain insight into the role of water in hydrous pyrolysis experiments. The results demonstrate that the substitution of hydrogen for deuterium in hydrocarbon products does not entail a homogeneous exchange reaction. Lewan used Woodford shale to perform hydrous/anhydrous hydrocarbon-generation simulation at 300 °C, 330 °C and 360 °C for 72 hours [3]. Helium was filled to increase pressure, once the sample was sealed. The analysis shows obvious differences between hydrous and anhydrous experimental products. Hydrous experiments were higher in hydrocarbon-generation potential and hydrocarbon yields, and kerogens were richer in hydrogen. Even lower Ro values appeared. By comparison, pressure-based methods are included in our experiments in addition to the influence of water. Although hydrocarbon components are very complex, their molecules mainly include C—H, C=C and the C=O bonds. According to current understanding, the participation of water in hydrocarbon generation may involve the following distinct processes: water reacts with carbonyls, alkyl carbons and free radicals to form hydrocarbons or intermediate products directly; it also causes olefin to hydrogenate and form saturated hydrocarbons [3]. (1) The interaction between water and aldehydes, esters and ketones with the carbonyl group takes place in near-critical conditions. Taking ester as an example, the chemical equation

(2) Water may directly react with alkyl carbons [17]. For example, alkanes with nine or less carbon atoms react with water under reservoir conditions (i.e., 100 to 150 °C and 40 MPa ) to form CO2 and alkanes with one less alkyl group: 25C9 H 20 + 2H 2O ⇔ 28C8H18 + CO 2

(3) Most hydrocarbon products contain free radicals, which also react with water molecules. By way of example, free radicals in alkyl react with water to form aldehydes and alcohols and provide hydrogen. Taking the generation of aldehyde as an example, the chemical equation is as follows: 3 C n H 2 n +1 + H 2 O ⇔ C n H 2 n O + H 2 2 (4) The conversion of olefins to alkanes has also been confirmed. Ethene may be converted to ethane in aqueous solution at 325 °C and 35 MPa [18]. Previous research shows that the reactions of water with alkyl and free radicals are thermodynamically favorable during the formation of oil. Although the mechanisms still need to be confirmed, it helps us to understand the complex chemical reactions of the organic matter in source rocks. Dehydrogenation takes place in some reactions described above, which provide hydrogen for organic matter. The fact that it is difficult to find free hydrogen in products from either underground or in-lab hydrous simulation, also proves the participation of hydrogen. The oxygenic negative ion can generate a new group or combine with carbon removed from organic matter, to form CO2 through a series of intermediate reactions. Due to the continuous degradation of organic matter, unsaturated chemical bonds increase. Given that foreign hydrogen serves as a supplement, water consumption occurs in the main hydrocarbon-generating period. It is closely related to carbon degradation and transformation in kerogen and soluble organic matter. The presence of water is conducive to kerogen

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degradation and further transformation of bitumen and hydrocarbons. Because source rocks differ as regards hydrocarbon-generation mechanisms during the various stages of evolution [19], their water-consumption characteristics obviously are also different. The hydrocarbon-generation process entails a continuous step-wise sequence, in which heavy oil is formed during the early stage, followed by the gradual generation of light oil and gas hydrocarbons from degradation, and the progressively increasing hydrogen-enrichment of products. Given the effects of water on hydrocarbon generation and the evolutionary patterns displayed in the source rocks in the Dongying Sag, the process of water consumption can correspondingly be divided into 3 stages, i.e. a low-mature, mature and high-mature stage. In the low-mature stage (Ro<0.5%), source rocks may generate some immature oil, which, as such, is closely related to the degradation of soluble organic matter, mainly involving the removal of heteroatomic groups. The addition of water increases the saturation of the chemical bonds. The gas generated, mainly contains CO2. In the mature stage (Ro = 0.5% – 1.2%), being the major hydrocarbon-generation period, the degradation of organic matter is dominated by dealkylation, which features the rapid loss of carbon element in kerogen and the further transformation of some of the products. As a result of much of the hydrogen in kerogen having been used, a part of the hydrogen supplement derived from water, is still needed for the products - heralding the main stage of water consumption. In the high-mature stage (Ro > 1.2%), the degradability of kerogen is almost depleted with most of the valid carbon having been used up, although some hydrocarbons, primarily condensate and wet or dry gas, are still being generated. The limited degradability of organic matter during this stage results in low water-consumption. In some simulation experiments, monatomic carbon reacts with water to form methane at high temperatures and at an advanced evolutionary stage [20], but underground temperatures are much lower than in laboratories, source rocks are mostly in tight compaction when reaching high evolutionary stage, it is difficult for them to provide enough hydrogen. 3.2

the macromolecular liquid products, C1—C5 hydrocarbons and non-hydrocarbon gases (N2, CO2), the products contain C6—C14 volatile hydrocarbons, which are difficult to preserve and measure during the sampling process. The essence of organic-matter degradation is the process of elemental redistribution in all products. The addition of water can change the amounts of hydrogen and oxygen. Although CO2 gas can be generated by inorganic carbonate, its yields have been proven to be small at low temperatures during experimental simulation [20−22]. Carbon can therefore be selected as reference for purposes of compensation calculations in respect of light volatile hydrocarbon-yields, using the formula: Cv = Co − Cr − Ce − Cg (3) The amounts of volatile oil are obtained by multiplying the carbon amount d by the conversion coefficient (1.22). The sum of the residual hydrocarbon-generation potential (S1+S2) in rock samples and the hydrocarbon loss (expelled liquid oil + volatile oil + gas hydrocarbon), is taken as the total hydrocarbon-generation potential. In water-hydraulic simulation performed on the Well Bin 338-6 sample, the total hydrocarbon-generation potential went up by 4.22% at 275 °C, 12.34% at 300 °C, 27.69% at 325 °C, and then went down at 350 °C. With the change in the extent of hydrocarbon-generation, the carbon-degradation rate varies gradually. The hydrocarbongeneration increment is proportional to the carbon-degradation rates (Fig. 4). Carbon-degradation rates have close relationships with organic-matter types and maturities. In relation to the same type of organic matter, the higher the maturity, the higher the carbon-degradation rate. For the same maturity, the better the organic type, the higher the carbon-degradation rate [23] . The carbon-degradation rate D is calculated by the following formula: D = Cd Co (4) Cd = 0.083M d

(5)

As indicated above, carbon-degradation rates can be calculated by applying Formula 4. Increments in hydrocarbon-generation potential can be obtained from Fig. 4. Furthermore, water consumption by unit of organic carbon can be determined by applying Formula (2), enabling the final calculation

The extent and zones of water consumption

It is difficult to accurately measure all kinds of microscopic hydrocarbon components due to the complex composition of underground hydrocarbons. Some understanding of the chemical reactions involved, is still at the qualitative level. It is difficult to measure the reactants and products via chemical equations, but material variation can be used as an important basis for estimating water consumption. Based on the exchange of hydrogen between water and organic matter, water consumption can be estimated by applying the following formula: M w = 9 H ΔM p (2)

Formula (2) shows that the amount of hydrocarbon generated has an important effect on water consumption. Besides

Fig. 4 Relationship between the degradation rate of kerogen-carbon and hydrocarbon-potential increment

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Fig. 5

Water-consumption rate during hydrocarbon generation vs. depth

of water-consumption rates for different evolutionary stages (Fig. 5). Water-consumption in the Upper Es4 source rocks occurs earlier than in the Lower Es3 and the Middle Es3 source rocks due to the generation of immature oil. The water-consumption rates in the Upper Es4 and the Lower Es3 source rocks are higher than in the Middle Es3 source rocks, due to their higher hydrocarbon-generation yields. The high water-consumption rate in the Upper Es4 source rocks occurred below 2 500 m, that in the Lower Es3 source rocks, below 3 000 m and in the Middle Es3 source rocks, below 3 300 m. Based on the burial history, distribution pattern, organic geochemical features and water-consumption rates of the major source rocks in the Dongying Sag, the water-consumption zones and the amounts of water consumed during hydrocarbon generation, are analyzed. Taking a 200 m depth interval as a unit for purposes of calculation, the calculation commenced from a depth of 1 000 m. The statistics relating to source rock volumes and the extent of organic carbon show the following relationships: water consumption in each calculation unit is equal to the product of the source rock volume, density, organic-carbon content and the water-consumption rate of the unit depth. The distribution and amounts of water consumption can be seen after calculation (Fig. 6). The total water consumption in the Upper Es4 source rocks is 40.77 ×108 t. Because the Upper Es4 source rocks enter the hydrocarbon-generation threshold earlier, the depth at which initial water consumption occurs, is shallow (1 500 m). Water consumption during hydrocarbon generation is still at a high level below 4 000 m, while peak consumption occurs in the 3 300 – 3 700 m depth interval. The total water consumption of the Lower Es3 source rocks is 34.63 ×108 t. It mainly distributes in the 1900 – 4100 m depth interval, the peak consumption occurs in 3100 – 3500 m. The total water consumption in the Middle Es3 source rocks is 29.27 ×108 t. It mainly occurs in the 1 900 – 3 700 m depth interval and concentrates at about 3 000 m. In comparison, the Upper Es4 source rocks have a larger water-consumption span as well as a larger total amount of water consumption. The maximum intensity of water consumption appears in the Lijin and Minfeng sub-sags in which the source rocks are deeply buried. The three sets of source rocks are distinct in both water-consumption zones and peaks. Besides differing degradation rates, the buried depth of source

Fig. 6 Relationship between water consumption in major source rocks and depth, in the Dongying Sag

rocks also plays an important role in the differences in water consumption. The sequence of water-consumption periods is consistent with the depth sequence of the source rocks.

4 Geological significance of water consumption in hydrocarbon generation 4.1

Influence of water consumption on pore fluid

During hydrocarbon generation, water molecules are absorbed while CO2 is released. As a result, the hydrocarbon-generation potential increases. The volume of the consumed water can be calculated by the following formula: Vw = M w ρ w = 9 H ΔM p ρ w (6) The volume increments resulting from added hydrocarbon due to water consumption, are calculated as follows: ΔVp = ΔM p ρ p (7) The products of early hydrocarbon-generation mainly contain oil. Assuming that the water density is 1 g/cm3, the average hydrogen-content in oil is 12% and the oil density is 0.85 g/cm3, the relation is established as follows: Δ Vp ΔM p ρ p = ≈ 1.1 (8) Vw 9 H ΔM p ρ w

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Formula (8) shows that ∆Vp > Vw. Apparently, water consumption during hydrocarbon generation constitutes a process of fluid-volume increase. Even if only liquid oil is formed, the fluid volume can be increased by about 10% compared with the water consumed. This is favorable to fluid-pressure increase and hydrocarbon-expulsion and migration. The above analysis shows that water consumption can directly improve the hydrocarbon yield. Given the increments in hydrocarbon-generation potential, oil saturation can be improved by about 10% – 20% during the main stage of hydrocarbon generation. This may consequently cause the source rocks to reach hydrocarbon-expulsion thresholds earlier and increase hydrocarbon-expulsion efficiency [24]. 4.2 Relationship between water consumption in hydrocarbon generation and diagenesis

In general, diagenesis can be divided into an early diagenetic stage and a late diagenetic stage. The early diagenetic stage comprises Periods A and B, whereas the late diagenetic stage consists of Periods A, B and C. Each of these periods has undergone a different evolutionary process, displaying the formation of different diagenetic evolutionary sequences [1]. Mineral alteration, which mainly occurs in Period A of the late diagenetic stage, requires water. The alteration process mainly centres on kaolinization and chloritization. Period A of the late diagenetic stage of the Palaeogene in the Jiyang Depression generally occurs at depths of 2 200 – 3 300 m. Considering water from montmorillonite dehydration and compaction being offset by consumed water during hydrocarbon generation, the effective diagenetic water-consumption interval may lie in a depth range of 2 500 – 3 500 m. By comparison, water consumption in mudstone during hydrocarbon generation lies in a 1 500 – 4 000 m range, larger in scope than that of sandstone. Hydrocarbon source rocks in the study area were developed in the center of the basin. Water-consumption peaks occur at depths ranging from 2 500 to 4 000 m. Both sandstones and mudstones are in water consumption at a depth of 2 500 – 3 500 m, at which most source rocks also enter expulsion thresholds. 4.3

Water consumption and oil-gas accumulation

Oil-gas reservoirs in the Dongying Sag concentrate within a depth range of 2 500 – 3 500 m. This applies especially to low-permeability turbidite-sand reservoirs, which, in turn, is consistent with the depth range of the diagenetic water-consumption interval. Deep reservoirs (2 800 – 3 500 m) mainly contain oil, while medium-deep reservoirs (2 000 – 2 800 m ) mainly contain water. In conjunction with an analysis of the burial history, it can be concluded that reservoir formation all occurred during the water-consumption stage. This took place especially during the sedimentary period of the Minghuazhen Formation, when most oil-gas reservoirs were finally formed in the Dongying Sag and even the entire Jiyang Depression itself, and the sandstones were at the peak of effective water consumption. At the same time, abnormal

pressure occurred in the mudstones, with formation water mostly being preserved in a closed system. Rock compaction progressed slowly, with little water being expelled. Clay minerals dehydrated only slightly, due to weak alteration. Water consumption obviously took place during hydrocarbon generation, such being the effective water- consumption zone. Differing from mudstone, water consumption in sandstones causes a decrease in fluid volume and pore pressure, which tends to increase the pressure difference and to improve reservoir properties, and makes the injection of hydrocarbon from source rocks to sands easier. Water consumption during hydrocarbon generation occurs during the main hydrocarbon-generation periods, and the main water-consumption intervals in sandstones are shallower than those in mudstones, enhancing the existent vertical-pressure gradient. The addition of water to organic matter increases the potential for hydrocarbon generation, and then finally improves the efficiency of hydrocarbon generation and expulsion, respectively. As regards the evaluation of source rocks, attention should in the future be paid to water consumption. Water consumption during hydrocarbon generation in the Dongying Sag is not only conducive to the formation of-self-generating and self-preserving reservoirs in the oil window, but also to the formation of lower-generating and upper-preserving reservoirs, in that it increases the pressure gradient between deep and shallow layers.

5

Conclusions

Based on experimental simulation and geological analysis, the geochemical effects of water consumption during hydrocarbon generation were studied. During hydrocarbon generation, organic matter can consume water, such then constituting a hydrogen supplement which serves to increase the total hydrocarbon-generation potential. Water consumption mainly occurs in the main hydrocarbon-generation stage, accompanied by the degradation of organic matter. Its influence on kerogen mainly manifests in the activation of invalid carbon and hydrogenation in the formed products, thus generating more hydrogen-rich hydrocarbons or transitional compositions The process of water consumption can be divided into several clear stages. Based on the water-consumption mechanisms, a method for the calculation of water consumption during hydrocarbon generation has been put forward. As regards the main source rocks in the Dongying Sag, calculation results show that both the extent of water consumption and the water-consumption zone in the Upper Es4 source rocks, are the largest. Those of the Es3 Member are smaller, with the amount of consumption in the Lower Es3 being larger than that in the Middle Es3. Water consumption during the process of hydrocarbon generation has a significant effect on oil-gas migration and accumulation. Although some pore water is consumed, the extent of hydrocarbon generation increases. Water consumption during hydrocarbon generation accordingly results in an increase

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in the overall fluid volume. The process raises pore-fluid pressure and oil saturation. Matched against the water-consumption interval of sandstones, the improvement can effectively increase the pressure difference between reservoirs and source rocks, which is conducive to the formation of self-generating and self-preserving reservoirs and of lower-generating and upper-preserving reservoirs.

[8] [9]

[10]

Nomenclature GI—hydrocarbon generation potential index, mg/g; S1—volatile hydrocarbon content, mg/g; S2—thermal cracking hydrocarbon content, mg/g; TOC—total organic carbon content, %; Mw—water consumption amount in hydrocarbon generation, g; ∆Mp—hydrocarbon generation potential increment, g; H—hydrogen content in hydrocarbons, %; Cv—carbon amount of volatile hydrocarbon at a certain stage, g; Co—carbon amount of the original sample, g; Cr—carbon amount of residual sample at a certain stage, g; Ce—carbon amount of expelled oil at a certain stage, g; Cg—carbon amount of generated gas (including gas hydrocarbons and CO2) at a certain stage, g; Cd—carbon amount due to degradation, g; Md—total amount of generated hydrocarbons, g; Vw—water consumption volume, mL; ρw—water density, g/cm3; ∆Vp—hydrocarbon volume increments caused by water consumption, mL; ρp —hydrocarbon density, g/cm3.

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