water consumption in hydrocarbon generation and its significance to reservoir formation

RESEARCH PAPER

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.

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.

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 re-lationship between water consumption during hydrocarbon generation and hydrocarbon migration and reservoir formation. The simula-tion 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 hy-drocarbon 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 Mem-bers are smaller. Water consumption affects hydrocarbon migration and accumulation by increasing organic carbon degradation rate to in-crease fluid volume. Pore fluid pressure and oil-bearing saturation are consequently increased. The matching relationship between wa-ter-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 reser-voirs.

Key words: hydrocarbon generation; water consumption; reservoir formation; simulation; Dongying Sag

Introduction

Widely occurring underground in sedimentary rocks, for-mation water is not only the carrier of fluid potential that con-trols the dynamics of petroleum migration, but also plays a role in diagenesis as the reaction medium for a variety of re-actions. Research shows that "water consumption" caused by mineral alteration during diagenesis, leads to substantial re-duction in formation water, leaving reservoirs in a low-pres-sure 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 in-organic minerals and formation water. The catalytic effect of inorganic minerals on hydrocarbon generation has been com-monly 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 dis-solved 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 wa-ter-consumption mechanisms and the extent thereof during hydrocarbon generation, with a view to revealing the rela-tionships 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

WANG Yongshi et al. / Petroleum Exploration and Development, 2013, 40(2): 259–267

− 260 −

Table 1 Geochemical parameters of the simulation samples

Experimental number

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

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 re-spectively 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 se-quences. The geochemical characteristics of the samples used for simulation, are shown in Table 1. The organic-carbon con-tent 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 ex-perimental conditions, and referred to as the “Experimental simulator of oil-gas generation and migration”. The auto-claves 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 hydrau-lic pressure (B1) and simulation under plunger-exerted pres-sure (B2). Because a large amount of samples would be re-quired for the experiments, the samples for the two circum-stances 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 con-tact) 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 experi-ment was designed stepwise as follows: At first, apply pres-sure 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 tempera-ture 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 re-sidual 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 simula-tion 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 hydro-carbon generation-potential index (GI) is an effective indica-tor to denote hydrocarbon-generation capacity, its quantitative expression being as follows:

1 2 100%S SGITOC+

= × (1)

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 hydro-carbon-expulsion threshold is reached, GI represents the original hydrocarbon-generation capacity. After entering the threshold, GI represents the remaining hydrocarbon genera-tion-capacity.

In the 3 sets of source rocks from the Dongying Sag, the Upper Es4 and the Lower Es3 Members contain typical TypeⅠorganic matter, while the Middle Es3 Member, being com-


− 261 −

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 deposi-tional environments, the hydrocarbon-generation and expul-sion 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-expul-sion thresholds both occur at about 3 000 m. On natural evo-lution profiles, the GI increases after reaching the hydrocar-bon-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 signifi-cantly (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 (chloro-form 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


− 262 −

Table 2 Test results of simulated products

Tempera-ture/°C

Experimental number

Chloroform extract “A”/%

Expelled oil/%

Ro/% S1/(mg·g−1) S2/(mg·g−1) TOC/% Gas yield/ (mg·g−1)*

B1 1.503 6 0.017 6 0.36 2.70 57.80 8.81 7.13 275

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 10.47 300

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 14.19 325

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 26.56 350

B2 8.513 4 0.279 9 0.61 12.02 57.39 9.95 14.46

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 plunger- applied 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 gener-ated and less hydrocarbons expelled, which suggests that hy-drocarbon-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 pe-riod, while the decreasing stage is consistent with the hydro-carbon-expulsion period, once the peak of hydrocarbon gen-eration 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 hy-drocarbon-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 hydrocar-bon expulsion, invalid carbon content displayed a weak de-creasing trend, while hydrocarbon-generation potential showed only slight change. The comparison between the hy-drocarbon-generation potential and the carbon-conversion rate of the two types of simulations shows that added water par-tially 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 indi-cates 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.


− 263 −

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 Water-consumption mechanism

The changes in GI constitute both a comprehensive reflec-tion and effective evidence of water consumption during hy-drocarbon generation. Previous geochemical analysis has di-rectly confirmed that water participates in hydrocarbon gen-eration [3,10−16]. It remains, however, difficult to make a quan-titative 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 homoge-neous exchange reaction. Lewan used Woodford shale to per-form 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 an-hydrous 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 di-rectly; 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

is as follows:

(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:

9 20 2 8 18 225C H 2H O 28C H CO+ ⇔ + (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 alco-hols and provide hydrogen. Taking the generation of aldehyde as an example, the chemical equation is as follows:

2 1 2 2 23C H H O C H O H2n n n n+ + ⇔ +

(4) The conversion of olefins to alkanes has also been con-

firmed. Ethene may be converted to ethane in aqueous solu-tion at 325 °C and 35 MPa [18].

Previous research shows that the reactions of water with alkyl and free radicals are thermodynamically favorable dur-ing 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. De-hydrogenation 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 under-ground or in-lab hydrous simulation, also proves the partici-pation 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, unsatu-rated 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 or-ganic matter. The presence of water is conducive to kerogen


− 264 −

degradation and further transformation of bitumen and hy-drocarbons.

Because source rocks differ as regards hydrocarbon-genera-tion mechanisms during the various stages of evolution [19], their water-consumption characteristics obviously are also different. The hydrocarbon-generation process entails a con-tinuous 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 pro-gressively increasing hydrogen-enrichment of products. Given the effects of water on hydrocarbon generation and the evolu-tionary 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-ma-ture 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 dealkyla-tion, which features the rapid loss of carbon element in kero-gen 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 wa-ter 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 hydrocar-bons, 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 simula-tion 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 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 ex-change of hydrogen between water and organic matter, water consumption can be estimated by applying the following for-mula: w p9M H M= Δ (2)

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

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 redis-tribution 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 simula-tion [20−22]. Carbon can therefore be selected as reference for purposes of compensation calculations in respect of light volatile hydrocarbon-yields, using the formula: v o r e gC C C C C= − − − (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 hydro-carbon-generation potential. In water-hydraulic simulation performed on the Well Bin 338-6 sample, the total hydrocar-bon-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 hydrocarbon- generation increment is proportional to the carbon-degrada-tion rates (Fig. 4). Carbon-degradation rates have close rela-tionships 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 fol-lowing formula: d oD C C= (4) d d0.083C M= (5)

As indicated above, carbon-degradation rates can be calcu-lated by applying Formula 4. Increments in hydrocarbon-ge-neration potential can be obtained from Fig. 4. Furthermore, water consumption by unit of organic carbon can be deter-mined by applying Formula (2), enabling the final calculation

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


− 265 −

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 wa-ter-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 oc-curred 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 ma-jor source rocks in the Dongying Sag, the water-consumption zones and the amounts of water consumed during hydrocar-bon generation, are analyzed. Taking a 200 m depth interval as a unit for purposes of calculation, the calculation com-menced 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 calcu-lation 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 hydro-carbon-generation threshold earlier, the depth at which initial water consumption occurs, is shallow (1 500 m). Water con-sumption 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 con-sumption 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 ab-sorbed while CO2 is released. As a result, the hydrocar-bon-generation potential increases. The volume of the con-sumed water can be calculated by the following formula: w w w p w9V M H Mρ ρ= = Δ (6)

The volume increments resulting from added hydrocarbon due to water consumption, are calculated as follows:

p p pV M ρΔ = Δ (7)

The products of early hydrocarbon-generation mainly con-tain oil. Assuming that the water density is 1 g/cm3, the aver-age hydrogen-content in oil is 12% and the oil density is 0.85 g/cm3, the relation is established as follows:

p p p

w p w

1.19

V MV H M

ρρ

Δ Δ= ≈

Δ (8)


− 266 −

Formula (8) shows that ∆Vp > Vw. Apparently, water con-sumption 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 in-crease and hydrocarbon-expulsion and migration.

The above analysis shows that water consumption can di-rectly improve the hydrocarbon yield. Given the increments in hydrocarbon-generation potential, oil saturation can be im-proved by about 10% – 20% during the main stage of hydro-carbon 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 diage-netic 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 be-ing 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 con-sumption 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 wa-ter-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 analy-sis 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 min-erals dehydrated only slightly, due to weak alteration. Water consumption obviously took place during hydrocarbon gen-eration, 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 res-ervoir properties, and makes the injection of hydrocarbon from source rocks to sands easier. Water consumption during hydrocarbon generation occurs during the main hydrocar-bon-generation periods, and the main water-consumption in-tervals 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 hydro-carbon generation, and then finally improves the efficiency of hydrocarbon generation and expulsion, respectively. As re-gards the evaluation of source rocks, attention should in the future be paid to water consumption. Water consumption dur-ing hydrocarbon generation in the Dongying Sag is not only conducive to the formation of-self-generating and self-pre-serving 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 hydro-carbon generation were studied. During hydrocarbon genera-tion, organic matter can consume water, such then constituting a hydrogen supplement which serves to increase the total hy-drocarbon-generation potential. Water consumption mainly occurs in the main hydrocarbon-generation stage, accompa-nied 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 composi-tions

The process of water consumption can be divided into sev-eral clear stages. Based on the water-consumption mecha-nisms, a method for the calculation of water consumption during hydrocarbon generation has been put forward. As re-gards 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 gen-eration has a significant effect on oil-gas migration and accu-mulation. Although some pore water is consumed, the extent of hydrocarbon generation increases. Water consumption dur-ing hydrocarbon generation accordingly results in an increase


− 267 −

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

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 consump-

tion, mL; ρp —hydrocarbon density, g/cm3.

References

[1] Zhang Shanwen. “Water consumption” in diagenetic stage and its petroleum geological significance. Acta Sedimentologica Sinica, 2007, 25(5): 701–707.

[2] Peng Chuansheng. Distribution of favorable lacustrine car-bonate reservoirs: A case from the Upper Es4 of Zhanhua Sag, Bohai Bay Basin. Petroleum Exploration and Development, 2011, 38(4): 435–443.

[3] Lewan M D. Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta, 1997, 61(17): 3691–3723.

[4] Huang Difan, Qin Kuangzong, Wang Tieguan, et al. Formation and hydrocarbon generation mechanism of coal-derived oil. Beijing: Petroleum Industry Press, 1995: 1–82.

[5] Qin Jianzhong, Liu Jingwang, Liu Baoquan, et al. Hydrocar-bon yield and geochemical parameters affected by heating time and added water amount in the simulation test. Petroleum Geology & Experiment, 2002, 24(2): 152–157.

[6] Wang Ruifei, Shen Pingping, Zhao Liangjin. Diagenesis of deep sandstone reservoirs and a quantitative model of porosity evolution: Taking the third member of Shahejie Formation in the Wendong Oilfield, Dongpu Sag, as an example. Petroleum Exploration and Development, 2011, 38(5): 552–559.

[7] Zhou Xinke, Xu Huazheng, Li Lili. Phase change of formation water and the formation of deep basin gas: A case from the Upper Paleozoic, Ordos Basin. Petroleum Exploration and

Development, 2012, 39(4): 452–457. [8] CNPC. Transmission light-fluorescent light kerogen maceral

appraisal and type partition method. SY/T 5125-1996, 1997. [9] Dong Quanhui, Wang Chong. Progress in research on the ac-

tion between the organic compound and the superheated water during aquathermolysis of heavy oil. Inner Mongolia Petro-chemical Industry, 2009, 35(22): 120–125.

[10] Rowland S J, Aareskjold K, Xuemin G, et al. Hydrous pyroly-sis of sediments: Composition and proportions of aromatic hydrocarbons in pyrolysates. Organic Geochemistry, 1986, 10: 1033–1040.

[11] Comet P A, McEvoy J, Giger W, et al. Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogen: A comparative study us-ing a biological marker approach. Organic Geochemistry, 1986, 9(4): 171–182.

[12] Eglinton T I, Rowland S J, Curtis C D, et al. Kerogen-mineral reactions at raised temperatures in the presence of water. Or-ganic Geochemistry, 1986, 10: 1041–1052.

[13] Hoering T C. Thermal reactions of kerogen with added water, heavy water, and pure organic substances. Organic Geochem-istry, 1984, 5(4): 267–278.

[14] Schimmelmann A, Lewan M D, Wintsch R P. D/H isotopic of ratios of kerogen, bitumen, oil, and water in hydrous pyrolysis of source rocks containing kerogen types Ⅰ, Ⅱ, and Ⅲ. Geochimica et Cosmochimica Acta, 1999, 63(22): 3751–3766.

[15] Schimmelmann A, Boudou J P, Lewan M D, et al. Experi-mental controls on D/H and 13C/12C ratios of kerogen, bitu-men and oil during hydrous pyrolysis. Organic Geochemistry, 2001, 32(8): 1009–1018.

[16] Mastalerz M, Schimmelmann A. Isotopically exchangeable organic hydrogen in coal relates to thermal maturity and mac-eral composition. Organic Geochemistry, 2002, 33(8): 921–931.

[17] Helgeson H C, Knox A M, Owens C E, et al. Petroleum, oil field waters, and authigenic mineral assemblages: Are they in metastable equilibrium in hydrocarbon reservoirs?. Geo-chimica et Cosmochimica Acta, 1993, 57(14): 3295–3339.

[18] Seewald J S. Evidence for metastable equilibrium between hydrocarbons under hydrothermal conditions. Nature, 1994, 370: 285–287.

[19] Huc A Y, Durand B, Roucache J, et al. Comparison of three series of organic matter of continental origin. Organic Geo-chemistry, 1986, 10: 65–72.

[20] Zhang Shanwen, Zhang Linye, Bao Youshu, et al. Formation fluid characteristics and hydrocarbon accumulation in the Dongying Sag, Shengli Oilfield. Petroleum Exploration and Development, 2012, 39(4): 394–405.

[21] Zhang Hengzhong. Regularity of decomposition temperature of carbonates. Nonferrous Metals, 1994, 46(3): 58–60.

[22] Li Linqiang, Jin Xiaohui, Lin Renzi. Simulation experiment of carbon dioxide forming in Dongpu Sag. Journal of Xi’an Shi-you University: Natural Science Edition, 2004, 19(4): 80–82.

[23] Baskin D K. Atomic H/C ratio of kerogen as an estimate of thermal maturity and organic matter conversion. AAPG Bulle-tin, 1997, 81(9): 1437–1450.

[24] Sun Hefeng, Zhou Xinhuai, Peng Wenxu, et al. Late-stage hydrocarbon accumulation and enrichment in the Huanghekou Sag, southern Bohai Sea. Petroleum Exploration and Devel-opment, 2011, 38(3): 307–313.

water consumption in hydrocarbon generation and its significance to reservoir formation

Documents