Hydrocarbon Generation in the Lacustrine Mudstones of theWenchang Formation in the Baiyun Sag of the Pearl River MouthBasin, Northern South China SeaYajun Li,† Zhenglong Jiang,*,‡ Shuang Liang,§ Junzhang Zhu,∥ Yuping Huang,∥ and Tiansi Luan‡
†School of Energy Resources, China University of Geosciences, Beijing 100083, China‡School of Marine Sciences, China University of Geosciences, Beijing 100083, China§PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China∥Shenzhen Branch of CNOOC, Ltd., Guangzhou 510240, China
ABSTRACT: The lacustrine mudstones of the Eocene Wenchang Formation are the primary source rocks of natural gas in theBaiyun Sag of the Pearl River Mouth Basin, China. The hydrocarbon generation kinetic parameters and the characteristics oflacustrine mudstones sampled from the Eocene Wenchang Formation at well LF13-2-1 were investigated on the basis of athermocompression simulation experiment in a closed system. In addition, combined with the burial history and paleothermalhistory, the gas generation process of the Eocene Wenchang Formation source rocks in the Baiyun Sag was investigated throughthe hydrocarbon generation equations established from parameters of hydrocarbon generation kinetics. The gas generationprocess of the Eocene Wenchang Formation lacustrine mudstones has three thermal evolutionary stages: the middle mature stage(39−30 Ma), the late mature stage (30−23.5 Ma), and the main gas generation stage (23.5−0.0 Ma). The main hydrocarbongeneration stage in the central area of the Baiyun Sag occurred from 24 Ma to 22 Ma, while the main hydrocarbon generationstage in the eastern and northern areas occurred at ∼15 Ma.
1. INTRODUCTION
The Pearl River Mouth Basin (PRMB) is located in thenortheast of the South China Sea (SCS). The Baiyun Sag(BYS) is the largest sag of the Zhu II Depression in the PRMB.The BYS underwent an interactive process of plate tectonicmovement between the Pacific plate, the Eurasia plate, and theIndian plate, and it was influenced by the spreading of theSCS.1 Thermal subsidence caused by a greatly thinnedlithosphere and active magmatism made the BYS a subsidenceand deposition center. The maximum residual Paleogenethickness is ∼8000 m.2 Recently, as a result of a series ofcommercial discoveries starting with wells LW 3-1-1 and PY30-1,3−5 the discovered petroleum reserves in the BYS have moregas (∼2000 × 108 m3) than oil (3500 × 104 m3).6 The BYS hasbecome a new focus for deepwater exploration with a hugehydrocarbon potential.7
As indicated by recent studies, the BYS has a verticalmigration pathway from deep source rock to a shallowreservoir.7,8 Therefore, it is critical to analyze the mechanismsof hydrocarbon generation of source rocks in the BYS. In thisstudy, based on a thermocompression simulation experiment,9
the chemical kinetic equations of the hydrocarbon generationof the Wenchang Formation source rocks were established, andthe hydrocarbon generation process was simulated using theBasinMod software.
2. GEOLOGICAL SETTING
The PRMB consists of two depression zones, separated bythree uplift zones: the North Uplift Zone, the NorthDepression Zone (consisting of the Zhu I and Zhu IIIDepressions), the Central Uplift Zone, the South Depression
Zone (consisting of the Zhu II and Chaoshan Depressions),and the Southern Uplift Zone.10 The BYS is the largest sag inthe Zhu II Depression, with an area of ∼15 000 km2 in a waterdepth of 200−2800 m beyond the slope-break zone of the SCS3
(see Figure 1).The northern SCS has experienced a transition, from a
Mesozoic active continental margin to a Cenozoic passivemargin. The BYS experienced three tectonic stages: (1)Paleogene rifting in three phases, which formed lacustrineand terrestrial-marine transitional strata; (2) a phase ofNeogene thermal subsidence; and (3) a phase of Pliocene-Quaternary neotectonic movement.11 From top to bottom, theBYS develops the Paleogene Formations of Shenhu, Wenchang,Enping, and Zhuhai, the Neogene Formations of Zhujiang,Hanjiang, Yuehai, and Wanshan, and the Quaternary Formation(see Figure 2). From the Palaeocene to the Early Oligocene,fluvial facies, lacustrine facies, and swamp facies have developedin the study area, and the sediments of the delta and open seahave been developing from the Late Oligocene to the present.Because of the expansion of Baiyun Lake, high-qualitylacustrine source rocks were widely developed during theperiod from the Eocene to the early Oligocene. The deeplacustrine source rocks of the Eocene Wenchang Formation, inparticular, are the major resource rocks with huge hydrocarbonpotential, since the deep lacustrine facies were developed in theprimary rifting stage.12,13
Received: September 8, 2015Revised: November 18, 2015Published: November 19, 2015
The bottom section of the Eocene Wenchang Formationdrilled in the well PY27-2-1 is composed of brown mudstonesinterbedded with grayish-white gravel-containing sandstones,
which indicates a river channel facies. The top section iscomposed of sandstones interbedded with dark gray mudstonesand contains coal-bearing source rocks in certain areas, whichindicates a swamp facies. The transitional coal-bearing sourcerocks are the major source of gas, as represented by the EnpingFormations in the PRMB.5 Meanwhile, the lacustrine sourcerocks of the Eocene Wenchang Formations were found in theLW4-1 tectonic zone.6,15 Compared with the seismic reflectioncharacteristics of the Zhu I Depression, the sequenceboundaries can be identified in the Eocene WenchangFormation (see Figure 3). The BYS trends NE-SW and iscomposed of multiple half-grabens and grabens from a series ofsecondary faults.14,16−20 Syn-rifting lacustrine sediments, i.e.,the Eocene Wenchang Formation, are extensively developed inthe BYS, which are the deep source rocks with typical features:
Figure 1. Regional location of the Baiyun Sag.1
Figure 2. Simplified lithostratigraphy and sea-level changes of thePearl River Mouth Basin.14
Figure 3. Sequence boundary identification of the Eocene Wenchangformation in the Baiyun Sag. [Legend: Tg, the bottom sequenceinterfaces of with the Paleogene Wenchang Formation; T80, the topsequence interfaces with the Paleogene Wenchang Formation; T70, thetop sequence interfaces with the Paleogene Enping Formation; F1, theinitial flooding surface of the Paleogene Wenchang Formation; and F2,the initial flooding surface of the Paleogene Enping Formation.]
moist climate, very thick strata, high deposition rate, and strongparallel seismic reflection facies.1
The seismic characteristics of the Eocene WenchangFormation in the center of the BYS are continuous medium-high amplitude, parallel and low-frequency,21 which are similarto the reflection features of deep lacustrine mudstones in theZhu I Depression. Low amplitude, continuous and parallelseismic reflection features are widely found in the top section ofthe Eocene Wenchang Formation, indicating shallow lacustrinefacies (see Figure 4a).22
The deep lacustrine strata are deposited in the center of theBYS, which has the tendency of becoming thinner toward bothsides of the northern slope and southern uplift (Figure 4b).Based on seismic reflections, the estimation of the maximumthickness of the Wenchang Formations is ∼6000 m. During thesedimentary period of the lowstand system tract, the lake basinwas limited by the northern fault zone and had a small area withdeep lacustrine facies in the center of the BYS. In thecorresponding period, shore-shallow lacustrine facies developednear the peripheral area of the BYS (Figure 4c). During thesedimentary period of the transgression and highstand systemtracts, the ancient lake basin expanded gradually, and deeplacustrine sedimentation expanded to the western fault zone. Inaddition, a relatively deep lacustrine facies developed east of theBYS (Figure 4d).
3. SAMPLES AND METHODS
3.1. Hydrocarbon Generation Kinetics Principle. Thekerogen pyrolysis produces both liquid hydrocarbons (C6−14and C15+) and gaseous hydrocarbons (C1 and C2−5). Thekinetics of kerogen pyrolysis have been successfully modeledwith either a first-order reaction or a system of parallel first-order reactions.23 The parallel reaction model is a moreextensive application, because kerogen is regarded as multi-compositional and the product composition is changedtremendously during pyrolysis.24,25 Based on the kineticprinciples of the primary cracking reaction proposed byEspitalie and Ungerer,26 a multicomponent kinetic model for
gas generation can be built for the thermal degradation ofkerogen by viewing it as a series of parallel first-order reactionswith different activation energies (Ei) and frequency factors(Ai) .
∑=X t X t( ) ( )i (1)
The gas generation of the ith reaction (at time point t) isXi(mL/g TOC) and can be expressed as
= − −X t X k t( ) {1 exp[ ( )]}i i i0 (2)
where
= −⎜ ⎟⎛⎝
⎞⎠k A
ERT
expi ii
(3)
where X is the total amount of gas generation at time point t(mL/g TOC), Xi0 the original amount of potential gasgeneration of the ith reaction at time point t (mL/g TOC),ki the Arrhenius reaction rate factor of the formation of the ithgas, t the time (h), Ei the activation energy (J/mol), Ai thefrequency factors (s−1), R the universal gas constant (R =8.31441 J/(mol K)), and T is the absolute temperature (K).The experimental data and theoretical calculations of the
natural gas generation potential were fitted and optimized usingBasinMod 1D software. Furthermore, the chemical kineticparameters, including the activation energy (Ei), the frequencyfactor (Ai), and the original potential of kerogen (Xi0), of anumber n of parallel first-order reactions were determined foreach kerogen component.27,28
3.2. Source Rocks of the Wenchang Formation. Thecomposition and structure of kerogen depends on the origin ofthe organic matter from which it has evolved, as well as itsdegree of evolution. Based on the carbon, hydrogen, andoxygen content, kerogens are usually classified into three maintypes (from the work of Tissot and Welte28). There have beentwo additional subclassifications: Type-II1 and Type-II2. Type-II1 is a kerogen of mixed origin that contains lipids with ahydrogen:carbon ratio (H/C) ranging from 1.0 to 1.4, anoxygen:carbon ratio (O/C) ranging from 0.10 to 0.15, a
Figure 4. Distribution of the lacustrine facies of the Wenchang Formation in the Baiyun Sag: (a) lowstand system tract of the Wenchang Formation,(b) transgressive and highstand system tracts of the Wenchang Formation, (c) seismic reflection characteristics of Line 1, and (d) seismic reflectioncharacteristics of Line 2.
hydrogen index (IH) ranging from 450 mg/g TOC to 650 mg/gTOC, and an oxygen index (IO) ranging from 25 mg/g to 50mg/g. Type-II2 is commonly rich in aromatic hydrocarbonswith a hydrogen:carbon ratio (H/C) ranging from 0.8 to 1.0, anoxygen:carbon ratio (O/C) ranging from 0.15 to 0.20, ahydrogen index (IH) ranging from 100 mg/g TOC to 450 mg/gTOC, and an oxygen index (IO) of >50 mg/g.The geochemical analysis of the crude oil, collected from the
core samples of the Zhujiang Formation north of the BYS,suggested it was from the deep Wenchang Formation.8,22 Inaddition, the high concentration of 4-methylsterane in the oilindicates that the Wenchang Formation in the Zhu IIDepression has developed the deep lacustrine source rocks,similar to the source rocks in the Zhu I Depression.21,22,29−32
The Wenchang Formation of the Huizhou Sag in the Zhu IDepression primarily developed grayish-black deep lacustrine toshore-shallow lacustrine mudstones, interbedded with graysandstone and coal bed in some areas. The organic matter inthe deep lacustrine source rocks is higher than that in theshore-shallow lacustrine source rocks (see Figure 5 and Table
1). The kerogen of the deep lacustrine mudstones are mainlyType-I and Type-II1, whereas the kerogen of the shore-shallowlacustrine mudstones are Type-II2 and Type-III (Figure 6). Inthe Huizhou Sag, the Wenchang source rocks discovered inwell LF13-2-1 are controlled primarily by a third-order/fourth-order sequence and characterized by a relatively highabundance of organic matter (Figure 7).33
The tectonic evolution, sedimentary environment, andsource rocks of the Zhu II Depression are well-correlatedwith the Zhu I Depression. The Wenchang source rocks in theBYS are primarily composed of deep lacustrine mudstones,which are similar to the source rocks in the Zhu I Depression
(see Figure 5), and the kerogen H/C ratios range from 1.0 to1.5 and are mostly 1.2. The microscopic examination showsthat the kerogen is primarily Type-II1, and the chromatographiccharacteristics of the pyrolysis gas show that the kerogen isType-I and Type-II1.
34 The Wenchang source rocks in the BYShave a relatively high hydrogen index (IH), in the range of 446−566 mg/g TOC.30 The TOC content of the Wenchang sourcerocks ranges from 0.75% to 24.84%, with a mean value of3.21%. The highest TOC value of 24.84% was found in thedeep lacustrine grayish-brown mudstones drilled from the wellPY27-2-1 (Table 2). The sources rocks of the WenchangFormation have high hydrocarbon generation potentials,ranging from 8.18 mg/g to 13.64 mg/g. The majority of thelacustrine source rocks of the Wenchang Formation are at thelate mature stage to main gas generation stage.
3.3. Thermocompression Simulation Experiment.There are three types of pyrolysis systems: an open system,an anhydrous closed system, and a closed system.24,35−56
Natural gas is more likely to be generated under closed andsemiclosed geological conditions than under open geologicalconditions.57,58 Thus, the experiment was performed underconfined system conditions using a closed gold-tube autoclavesystem at the Provincial Petroleum Laboratory of the ResearchInstitute of Petroleum Exploration & Development of thePetroChina North China (Huabei) Oilfield Company. Theexperimental instruments primarily consisted of the followingthree parts:
(1) the reaction kettle, which functions under a designedpressure of 19.6 MPa and is manufactured by DalianAutomatic Control Equipment Factory (Liaoning,China) (Model GCF-0.25L);
(2) the temperature controller with a digital regulator(Model XMT-131); and
Figure 5. Relationship between the hydrocarbon generation potential(S1 + S2) and the total organic carbon (TOC) of the WenchangFormation.33
Table 1. Characteristics of Different Facies of Source Rocks of the Wenchang Formationa
faciestotal organic carbon content,
TOC (%)hydrocarbon generation potential,
S1 + S2 (mg/g)highest pyrolysis peak temperature,
Tmax (°C)hydrogen index,
IH (mg/g)
deep lacustrine 3.06 (7.75−1.93)/17 12.33 (28.82−0.64)/17 438.41 (442−434)/17 410.35 (606−268)/17Shore-Shallowllacustrine
(3) the pyrolysis gas and gas condensate (or named lighthydrocarbon) collection−separation system that iscomposed of a liquid-nitrogen-cooled liquid-receivingtube to collect gas condensate and water, an ice-cooledspiral condenser pipe, and a graduated gas-collectioncylinder.
The pyrolysis was conducted under the standardized temper-ature ranges, acting time of constant temperature and heatingrate to obtain the experimental results, which can becomparatively analyzed with other studies.Existing studies were inclined to use different heating rates to
investigate the relationship between the temperature andhydrocarbon generation in pyrolysis experiments.23,30,59 How-ever, the maturity of the source rock is influenced by thecumulative effect of temperature and time, which is representedby the reflectance of vitrinite (Ro). Therefore, it is necessary toconduct a pyrolysis experiment to establish the relationshipbetween Ro and the hydrocarbon generation amount. In thisstudy, original samples were heated under different temper-atures to access the hydrocarbon generation amount duringdifferent thermal evolution stages, and Ro was measured afterthe heating experiments. The theoretical amounts of hydro-carbon generation and experimental results were then fittedusing basin modeling software (BasinMod 1D).One deep lacustrine mudstone of the Wenchang Formation
sampled from well LF13-2-1 (3126.5−3167.5 m) was selectedfor the thermocompression simulation experiment (Table 3).The adopted method in this experiment involves heating theoriginal samples separately at several separate temperaturelevels; that is, at each temperature value, the original samplesreact in an enclosed system and the presumed outcomes wouldnot be isolated from this system. Eight temperature values at200, 250, 275, 300, 325, 350, 400, and 500 °C are chosen toexplore the stages of thermal evolution of the source rock. Foreach simulated temperature, 80 g of the sample with the grainsize of 5−10 mm was added to the reaction kettle with
deionized water that amounts to a concentration of 10−20wt %. The reactor was first tested for leakage by repeating thetests by filling and vacuum pumping the nitrogen at a pressureof 4−6 MPa 3−5 times. The evacuated reactor was then heatedto a series of temperatures in a closed system. When thesimulated temperature is no more than 300 °C, the heatingtime can be 24, 48, or 72 h. In contrast, when the simulatedtemperature is no less than 325 °C, the heating time must belimited to <24 h. The evacuated reactor was maintained for 24h at a set temperature in a closed system. The gas and liquidproduced were collected for analysis once a reaction wascompleted.
4. RESULTS AND DISCUSSION4.1. Results. The thermocompression simulation experi-
ment conducted on the brown mudstone of the WenchangFormation from well LF13-2-1 has eight points within thetemperature range from 200 °C to 500 °C. The parameters,such as the gas generation rate, oil generation rate, and thehydrocarbon generation rate, were obtained from eachtemperature point (see Table 4).The original sample contains a certain amount of oil, which
made the initial oil generation ∼100 kg/t TOC (see Figure 8).As the maturity increased, the organic matter evolved from the
Figure 7. Thermal evolution profile of the well LF13-2-1 in the Huizhou Depression.
Table 2. Source Rock Characteristics of the Wenchang Formation in the PRMBa
aAverage given as the range of values (maximum to minimum)/number of samples. bHC - total hydrocarbon. cTOC - total organic carbon content,%. dIH - hydrogen index, mg/g. eTmax − highest pyrolysis peak temperature, °C.
Table 3. Description of Source Rock Samples Used in theStudy
parameter value
well LF13-2-1depth (m) 3126.5−3167.5 mcolor brownfacies deep lacustrinetotal organic carbon content, TOC (%) 2.02IH (mg/g TOC) 502Tmax (°C) 440reflectance of vitrinite, Ro (%) 0.56
thermal catalysis hydrocarbon-generation stage (the Ro valueranges from 0.7% to 1.2%) to the oil-cracking gas stage (the Rovalue ranges from 1.2% to 2.0%). The amount of oil generationfrom kerogens is less than the amount of secondary cracking ofoil, and the rate of oil generation decreased as the value of Roincreased beyond 0.94%. During the main hydrocarbon-generation stage, the range of the Ro values was between0.7% and 1.15%, and the rate of the hydrocarbon generationincreased dramatically, reaching a peak of 179.56 kg/t TOCwhen the Ro value reached 0.94%. During the main gas-generation stage (Ro values of 1.15%−1.25%), a large amountof gas was generated rapidly. The sample has experienced ahigh maturity (Ro values of 1.25%−2.0%), and the generationrate of hydrocarbons increased steadily. The organic mattertransformation in sedimentary basins is a result of the thermalbreakdown of macromolecular matter. Components with lowbond energy are first degraded to generate hydrocarbons atrelatively low temperatures and maturity, and the componentswith higher bond energy are cracked at relatively highermaturity levels. The kerogen of the source rock sample iscomposed of 71.43% Type-II1, 19.05% Type-II2, and 9.52%Type-I, with some highly stable bonds cracking at high maturitylevels, which slightly increases the oil generation rates when thevalue of Ro reaches 1.92%. A large amount of dry gas wasgenerated as the value of Ro increased beyond 2%. When thevalue of Ro reached 3.13% in the experiment, the maximum oilgeneration rate of 379.79 kg/t TOC and a maximum gasgeneration rate of 347.48 kg/t TOC were achieved (see Figure8). The characteristics of the sample’s hydrocarbon generationare in agreement with the patterns of Tissot and Welte.28
4.2. Hydrocarbon Generation Kinetic ParameterCorrection. The hydrocarbon generation process of thebrown mudstone of the Wenchang Formation from wellLF13-2-1 was simulated by using the BasinMod 1D software toobtain the optimal fit between the numerical simulation and the
thermocompression experimental data.60,61 The kinetic param-eters of the source rocks of the Wenchang Formation, such asthe activation energy, hydrocarbon generation potential, andcrude-oil cracking parameters, were obtained (Table 5). The
numerical simulation is consistent overall with the thermo-compression experimental results (see Figure 9), except for thehydrocarbon conversion rate at 84% (Ro = 1.92%), when thenumerical simulation is higher than experimental results.Components with low bond energy would be first degraded
to generate hydrocarbons at low temperatures and maturity. Incontrast, the components with high bond energy would onlydegrade to generate hydrocarbons when the temperature andmaturity are high.62,63 The range of activation energies is 46−63 kcal/mol, and the relatively broad distribution of thehydrocarbon generation potential is skewed negatively (seeFigure 10). A relatively broad distribution of activation energiesare directly related to the heterogeneity of kerogens.61 Theactivation energies of hydrocarbon generation of kerogen wereprimarily concentrated between 54 kcal/mol and 59 kcal/molwhen hydrocarbon generation potentials were more than 8%.Most hydrocarbon generation potentials of crude oil crackingare higher than 10% and distributed discretely. The highesthydrocarbon generation potential of crude oil cracking is 35%,when the activation energy reaches 53 kcal/mol (Figure 10).
Table 4. Production of Gas and Liquid during the Pyrolysis Process of the Samples
The components with high activation energy have larger initialhydrocarbon generation potentials, compared to the compo-nents with low activation energy, because the low-activation-energy components may vanish with increasing maturity.The organic matter in lacustrine kerogens is generally
described as very homogeneous, compared with other types oforganic matter. Lacustrine source rocks, such as the GreenRiver Formation, are usually associated with oil accumulation.The Green River shale is a typical lacustrine source rock, and itskerogen type is Type-I. The colors are generally light brown,dark brown, dark gray to black, and they are enriched in lipids.The characteristics of biomarkers of shale in the Green RiverFormation, including isoprenoid, steranes, and terpanes,showed that the organic matter is mainly from aquaticorganisms, including bacteria and algae, and the shale formedin a high reduction environment.64 The main organic mattercomposition is lacustrine planktonic algae and includes a smallamount of vitrinite, intertodetrinite, alginite, and bituminite.65
Existing research has discussed the kinetic model of thelacustrine Green River shale (see Figure 11 and Table 6) in theUinta Basin.66 The variability of chemical bonds of thelacustrine Green River shale is described with a singledominating activation energy of 57 kcal/mol, with a frequencyfactor of 1.8451 × 1013 1/m. This bond energy is characterizedas being thermally stable, compared to other types of organicmatter. The very limited distribution range of activation energysupports the homogeneous kerogens of the Green River shale.
Compared to the lacustrine Green River, the kineticevaluation of the Wenchang Formation results in a relativelybroad activation energy distribution and a lower frequencyfactor of 3.18 × 1011 1/m (minute). The type of kerogen in theWenchang Formation sample is primarily Type-II1 and contains3.52% sapropelinite, 17.03% exinite, 65.16% exinite−vitrinitehybrid (a mixture of a relatively high content of sapropeliniteand exinite in vitrinite), 13.18% vitrinite, and 0.37% inertinite.The lacustrine organic matter of the Wenchang Formationcontains aquatic organisms, such as planktonic algae. Deeplacustrine source rocks formed in a reduction environment, andshore-shallow lacustrine source rocks formed in a weakoxidation environment. The broad activation energy distribu-tion supports the high level of heterogeneity of the WenchangFormation source rock, which is clearly different from theGreen River shale.
4.3. Gas Generation Analysis. In this paper, the riftingand post-rift subsidence process of the BYS was simulated viaforward and inverse modeling. Numerous measured vitrinitereflectance values are used to examine the simulation resultsunder an extensional basin model.67−69 When the “simulated”Ro values have fitted the “measured” Ro values reasonably well,the geologic model provided the process of thermal evolution.Based on the thermal evolution simulation of four wells, thehydrocarbon generation process of the Wenchang Formationsource rock in the BYS was established by the obtained kineticparameters.70,71
4.3.1. Thermal History. Thermal history is important to thetiming, amount, and composition of hydrocarbon generation.72
The present-day heat flow is calculated from thermalconductivities of the rocks and the subsurface geothermalgradients, which are determined by measured temperatures.68
Paleoheat flow is affected by the tectonic evolution of the BYS,which dominated by lithospheric stretching during thePaleocene to early Oligocene rifting period and subsequentregional subsidence from Miocene to Quaternary.73 The heatflow measurements reported by Shi et al.74 were used toestablish the geological model in this study. Four simulationwells (marked in Figure 1) were established to analyze theburial and thermal evolution75−77 in the study area. TheWenchang Formation is divided into two sections to describethe burial and thermal evolution (Figure 12), which are the
Figure 9. Comparison of hydrocarbon conversion between the actualmeasurements of the thermocompression experiment data and thenumerical simulation results from well LH13-2-1.
Figure 10. Activation energy distributions of the hydrocarbongeneration of kerogen and crude oil cracking from well LF13-2-1.
Figure 11. Bulk kinetic parameters from the programmed pyrolysisfrom the Green River shale.66
Table 6. Rock-Eval TOC Characteristics of the Sample fromthe Green River Shale
lowstand system tracts and the lake transgression and highstandsystem tracts. The vitrinite reflectance data are well-correlatedbetween the modeling results (well 3) and the measured Rodata from the well PY33-1-1 (Figure 13).
The Wenchang Formation source rocks are currently in anovermature stage. The bottom of the Wenchang Formation(WC-bot) was in the middle mature stage, with Ro = 0.7%−1%during the period from 39 Ma to 32.5 Ma, and then evolvedinto the late mature stage, with the Ro = 1.0%−1.3% during theperiod from 32.5 Ma to 26 Ma. The Wenchang Formationfurther evolved into the main gas-generation stage, with the
value of Ro being >1.3% during the period from 26 Ma to thepresent. The top of the Wenchang Formation also hasexperienced multiple thermal stages, which are the middlemature stage, with Ro = 0.7%−1.0% from 32 Ma to 27.8 Ma,the late mature stage with Ro = 1.0%−1.3% during thedepositional period of the Zhuhai Formation (30−23.5 Ma),the main gas-generation stage with Ro = 1.3%−2.0% from 25.2Ma to 20.5 Ma, and the dry gas-generation stage, with Ro>2.0% from 15 Ma to the present (see Figure 14).Based on the degrees of evolution of different sections, the
thermal evolution of the Wenchang Formation can be dividedinto three stages: the middle mature stage, from 39 Ma to 30Ma; the late mature stage during the sedimentation period ofthe Zhuhai Formation (30−23.5 Ma); and the main gas-generation stage, from 23.5 Ma to the present.
4.3.2. Stage of Gas Generation. Overall, most of the organicmatter in the source rocks of the Wenchang Formation evolvedinto the stage of secondary crude oil cracking gas andovermature oil-type kerogen cracking gas. At present, thesource rocks of the Wenchang Formation generated a smallamount of oil in the eastern area (well 3) and the northern area(well 2). The source rocks of the Wenchang Formation aremainly in the stage of gas generation (Figure 15).The gas generation rates at the top of the Wenchang
Formation were simulated in four simulation wells (see Figure4). The central area (well 1) has a main gas-generation stage,which began from 23 Ma to 20 Ma. The hydrocarbongeneration peak occurred at ∼22 Ma, with a current gasgeneration of 430 mg/g TOC (Figure 15a). The main gasgeneration in the northern area (well 2) began 23.5 Ma until10.5 Ma, and the hydrocarbon generation peak occurred at ∼15Ma, with a current gas generation of 370 mg/g TOC (Figure15b). The main gas generation in the eastern area (well 3)began at ∼16.5 Ma to 10.5 Ma, and the hydrocarbon generationpeak occurred at ∼15 Ma, with a current gas generation of 280mg/g TOC (Figure 15c). In the central and western area, themain gas generation (well 4) began at ∼24 Ma to 14 Ma, andthe hydrocarbon generation peak occurred at ∼23 Ma, with acurrent gas generation of 430 mg/g TOC (Figure 15d). Thesimulation results of the BYS show that the hydrocarbongeneration peak (∼24−22 Ma) in the central area appearedearlier than that (∼15 Ma) in the eastern and northern areas.Because the BYS is located in the continental transitional
crust of the northern SCS, Cenozoic structural movementshave strongly deformed the sag, which is characterized by thegreatly thinned lithosphere and active magmatism.2 The BYSdeveloped thicker sedimentary sequences in the central areathan on both sides of the northern slope and southern upliftand also developed thick lacustrine source rocks in the EoceneWenchang Formation during the rifting stage. Starting at 23.8Ma, the BYS evolved from rifting to post-rifting thermalsubsidence and became the center of subsidence and depositionin the PRMB.2 The rising mantle under the BYS caused apartial melting of the upper mantle, which accommodatedextensional strain and caused nonfaulted vertical subsidence. Inthe center of the BYS, the tectonic subsidence andsedimentation rate is larger than other areas. Therefore, thethermal subsidence in BYS is the reason for the earlier peak inhydrocarbon generation in the central area. Furthermore, thetotal gas generation (∼430 mg/g TOC) in the central area wasapparently higher than that of the eastern (310 mg/g TOC)and northern (370 mg/g TOC) areas.
Figure 12. Temperature modeling and burial history in well 3.[Formation names: WC-bot = the bottom of the WenchangFormation; WC1 = the lowstand system tract of the WenchangFormation; WC2 = the lake transgression and the highstand systemtracts of the Wenchang Formation; EP1 = the lower part of the EnpingFormation; EP2 = the upper part of the Enping Formation; ZH = theZhuhai Formation; ZJ1 = the lower part of the Zhujiang Formation;ZJ2 = the upper part of the Zhujiang Formation; HJ1 = the lower partof the Hanjiang Formation; HJ2 = the upper part of the HanjiangFormation; and Yh-Q = the Yuehai Formation to Quaternary.] Thelocations of the modeled wells are shown in Figure 1.
Figure 13. Simulated Ro modeling matches with the measured Rodata in well 3.
Figure 14. Thermal and maturity history of the Wenchang Formation at well 1. (The locations of the modeled well are shown in Figure 1.[Formation names: WC-bot = the bottom of the Wenchang Formation; WC1 = the lowstand system tract of the Wenchang Formation; WC2 = thelake transgression and the highstand system tracts of the Wenchang Formation; EP1 = the lower part of the Enping Formation; EP2 = the upper partof the Enping Formation; ZH = the Zhuhai Formation; ZJ1 = the lower part of the Zhujiang Formation; ZJ2 = the upper part of the ZhujiangFormation; HJ1 = the lower part of the Hanjiang Formation; HJ2 = the upper part of the Hanjiang Formation; and Yh-Q = the Yuehai Formation toQuaternary.] The locations of the modeled wells are shown in Figure 1.
Figure 15. Cumulative amount and rate of gas generation of modeled wells. The locations of modeled wells are shown in Figure 1. System Epoch: E,eocene; O, oligocene; M, miocene; P, pliocene.
5. CONCLUSIONSIn this study, the thermocompression experimental data andthe simulation results were well fitted during the hydrocarbongeneration process, and the hydrocarbon generation character-istics of the Wenchang lacustrine mudstones were investigatedbased on a hydrocarbon generation kinetic equation.
(1) The thermocompression simulation experiment showedthat the brown mudstones from well LH13-2-1 had ahydrocarbon generation rate of 379.79 kg/t TOC and agas generation rate of 347.48 kg/t TOC.
(2) The activation energy of the brown mudstones from wellLH13-2-1 primarily ranges from 46 kcal/mol to 63 kcal/mol, and their hydrocarbon generation potentials had anegatively skewed distribution. The activation energies ofthe hydrocarbon generation of kerogen were concen-trated within the range from 54 kcal/mol to 59 kcal/mol,while the hydrocarbon generation potentials were >8%.The hydrocarbon generation potentials of crude oilcracking were mostly >10% and were distributeddiscretely with a peak at 35% when the activation energywas 53 kcal/mol.
(3) The thermal evolution of the Wenchang Formation canbe generally divided into three stages: the middle maturestage, during the period from 39 Ma to 30 Ma; the latemature stage, during the sedimentation period from 30Ma to 23.5 Ma; and the main gas-generation stage, from23.5 Ma to the present.
(4) The central area of the BYS has a gas generation peakfrom 23 Ma to 22 Ma, which was earlier than that (∼15Ma) of the northern area and the western area.
■ ACKNOWLEDGMENTSThis work was supported by the Techniques of IdentifyingHydrocarbon-Rich Depressions, Predicting Reservoirs andDetecting Hydrocarbons in the Northern Deep-Water Areaof the South China Sea (No. 2008ZX05025-03) and theNational Natural Science Foundation of China (Grant No.41030853).
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