Petroleum Geology And Basin Modeling Of The Chaluhe Basin,Yitong Graben

The Chaluhe Basin,which represents a part of the Yitong Graben in Jilin Province of China,is one of the petroferous basins in the graben(Luxiang and Moliqing basins).This basin has five subdivisions:Wanchang Structural high,Bo-Tai sag,Xinanpu sag,Liangjia Structure high and the Gudian slope.Based on apatite fission track analysis of some samples from the Chaluhe Basin and basin modeling study,it can be concluded that since the Oligocene(36.6 Ma),the Chaluhe Basin has undergone two episodes of uplift during 24.9–19.1 Ma and 6.9–4.9 Ma,with an average apparent exhumation rate to be 70.34 and since 21.8 Ma.The evidence from geological relationships such as geodynamics,volcanic activity,and stratigraphic division and correlation can also support the results of thermochronological analysis.Drilling,logging and paleontological studies reveal the Chaluhe Basin contains seven different sedimentary rocks formations from the Eocene to the Quaternary with scattered Neogene formation in the Cretaceous and Jurassic outcrops,overlying a pre-Tertiary basement.The distribution of the sediment thicknesses inside the Chaluhe Basin is deeper at the northern area of the basin,diminishing in depth towards the southern zone.Furthermore,the total thickness of the sediments ranges from 2000 m to 6000 m.So far,more than 50 wells have been drilled in the basin,to date the overall exploration within the Chaluhe Basin has not been very successful and only local hydrocarbon producing and shows have been discovered.In contrast,the Moliqing and Changchun oilfields have been found in the Moliqing and Luxiang basins in the SW part of Yitong Graben,with reserves of 3317×10⁴ t oil and 13.29×10⁸ m³gas.The Chaluhe Basin has the similar Cenozoic sediments and tectonic setting to the adjacent Moliqing and Luxiang basins.In addition,with widespread distribution of better source rocks and more favorable condition of hydrocarbon accumulation,suggest that the Chaluhe Basin should have a great potential of hydrocarbons and can be considered as the prime targets for the future hydrocarbon exploration within the Yitong Graben.The main purpose of this study is to research and comprehend the evolution of the Chaluhe Basin through petroleum geological analysis and basin modeling.Constructing a burial history model would serve as the basis upon which the geothermal and maturity model histories could be developed and analyzed.The objectives and contents include the evaluation of source rocks in the Chaluhe Basin:the thickness of dark shale,organic matter abundance,organic matter type and organic matter maturity.Likewise,reconstructing the histories of paleo-heat and paleo-geothermal heat flows are very important to the whole basin modeling concept because paleo-geothermal history is the key most objective factor in calculating source rock maturation and hydrocarbon generation potential history of source rocks in the Chaluhe Basin.It is also hoped that basin modeling of the Chaluhe Basin will increase the chance to find the commercial petroleum accumulation and decrease the cost of exploration in this area.The basin was filled with thick clastic deposits during the formation and expansion periods.Due to the systematic data of hydrocarbon source rocks in Liangjia structural high in this area,the existing data of the Chaluhe Basin in this area were used for reference.In the longitudinal direction of the Chaluhe Basin,there are three sets of source rock systems,namely,the Eocene Shuangyang,Sheling and Yongji formations,and the lithology is mainly dark mudstone.The thickness ratio of dark mudstone to formation thickness also varies in each member.The variation of the Sheling Formation is relatively the largest,the Shuangyang Formation is relatively small,and the thickness ratio of dark mudstone to formation thickness of the Yongji Formation is the largest,and the change is relatively small and stable.Overall,the accumulated thickness of dark mudstone in the strata is 0.5m-441m,and the thickness of dark mudstone in most strata accounts for more than 50%of the thickness of the strata,among which the content of the Yongji Formation is the highest,reaching more than 60%.In general,the total organic carbon content of dark mudstones of the Sheling,Shuangyang and Yongji formations in the Chaluhe Basin is mainly 0.6-1.0,followed by1.0-2.0.The source rocks are mainly medium-good grade source rocks.According to the pyrolysis data of rocks,the organic matter types of the Yongji Formation in the Chaluhe basin are mainly type II2.Type II1 also accounts for a considerable part,and the contents of type I and type III are very small;the organic matter types of the Sheling Formation are mainly type II1and type II2;the organic matter types of the Shuangyang Formation are mainly type II2;generally,the organic matter types of the Chaluhe basin are type II1,type II2and type III.Mainly,it is speculated that the kerogen is mainly derived from lake phytoplankton and terrestrial higher plants,and has the potential of oil generation and gas generation.There are six sets of sandstone reservoirs in the Wanchang structural belt of the Chaluhe Basin of the Yitong Graben,namely;the first,second and third members of the Shuangyang Formation,the first member of the Wanchang Formation,the first member of the Sheling Formation and the first member of the Yongji Formation.According to the porosity of reservoirs in this area,it can be divided into five ranges:ultra-low porosity 0%-5%,low porosity 5%-10%,medium porosity 10%-15%,high porosity 15%-20%,and ultra-high porosity 20%-25%.According to the study,the permeability of reservoir is relatively low overall,and the permeability of each formation decreases with the increase of depth.The average permeability of the Wan1 Member is about 2.0 x 10^-3um²,that of the She1 Member is about 3.1 x 10^-3um²,and that of the Shuang1 and Shuang2 members is about 10.1x10^-3um².The average permeability of the Shuang2 and Shuang3 members is about 1.2x10^-3um²,and that of the Yong1 Member is about 8.9 x 10^-3um².By comparing to the above porosity,it can be seen that the porosity of the formation is relatively good,but the permeability is generally low,indicating that the connectivity of the formation is not very good.The main reservoir sandstones in the Chaluhe Basin have undergone complex evolution during burial.The diagenesis mainly includes mechanical compaction,siliceousaccretion and cementation,carbonate metasomatism,sedimentation,transformation and dissolution of clay minerals.The Shuangyang,Sheling and Yongji formations are not only good reservoir,but also good source rock layer,and mostly develop in the center of the basin,with delta facies and deep lake facies.The growth fault,which is about 12 km long and extends in the plane,tends to be nearly east-west and northward.It is presumed that because of the development of this tensional-torsional fault in the central uplift area,the release of tectonic stress makes the reservoir physical properties in this area better than those in the surrounding oblique zone from the data point of view.However,the fault cut through the reservoir and the caprock in the area,resulting in poor reservoir forming conditions and damaging the existing oil and gas reservoirs.The reservoir property of the peripheral oblique zone is mainly medium porosity and low permeability.Therefore,the reservoir property of the peripheral oblique belt is better than that of the central uplift.Mudstone is the main caprock in this area.On the horizon,it is mainly composed of the Shuangyang,Sheling and Yongji formations of the Eocene series with four sets of regional caprocks overlying the Oligocene Qijia Formation.The petroleum system analysis of the Chaluhe Basin,though with structural damages in some tectonic areas,has generally a relatively good source rock distribution,reservoir,and sealing and caprock mechanisms in place for hydrocarbon exploration and exploitation.This paper bases the basin model calibration on the use of a Tmax based calculated vitrinite reflectance value,as well as the BHT and the pyrolysis Tmax.The inclusion of the calculated vitrinite reflectance will provide an additional method to verify the veracity of the models to describe the basin thermal conditions.Determining whether the basin has the appropriate conditions for hydrocarbon generation,expulsion,migration and accumulation for commercial exploration,the Basin Mod 1D and 2D,and Trinity software were employed.The reconstruction of the burial,thermal and maturity histories were modeled in order to evaluate the hydrocarbon potential using Basin Mod 1D 2000 software developed by Platte River Associates Inc.Four(4)well(WC1,Chang 25,Chang 7 and Chang 8)models were created by the use of geochemical,well log and further geologic data.Major 1D model input parameters comprise events or formations within the chrono-stratigraphy,deposition age,present and eroded thicknesses of formations and events,volumetric lithological mixes,kerogen types and kinetics and further geochemical parameters such as initial%TOC.The modeling results are also calibrated with measured vitrinite reflectance and borehole temperatures(BHT)of these four wells in the study area.The input data and materials for Basin Mod 2D are 1D well data,seismic lines 2,5 and 7,map coordinates,reference location of all data including faults,names and age ranges of all layers,lithologic characteristics of facies within the layers.The input parameters for Basin View are Basin Mod 1D transient heat flow values.The Trinity software was adopted for the maturity basin view modeling and the 3D fluid potential models.Trinity is all-purpose interactive map-based petroleum system analysis,prospect risking toolkit designed for quickly integrate data from seismic interpretation and any other geological data to integrate the geological history with hydrocarbon generation and migration to quantify critical parameters and charge risks for petroleum prospects or exploration blocks.Other required input parameters for Trinity such as the hydrogen index(HI),total organic carbon content(TOC),top seal capacity,estimated geothermal gradient and heat flow data and structural maps were obtained from regional dataset.The fluid potential timing period was decided on according to Feng et al.,2009 who identified the Late Eocene to Middle Oligocene(about 29Ma),the Early-Middle Miocene(about 10Ma)and the Quaternary(0Ma)are the three important reservoir-forming periods in the Chaluhe Basin.The result obtained after the petroleum geological analysis and basin modeling,reveals that heat flow was computed from the heat generation history model and was estimated to be around the average of 71.8 m W/m².The highest quantities of hydrocarbon generated from the Shuangyang Formation source rock presently is within the wells Chang 25 and Chang 8,reached 132.32(oil:39.49,gas:92.83)mg/g TOC and 116.1(oil:35.85,gas:80.25)mg/g TOC respectively,and the hydrocarbon generation amount of the Shuangyang Formation source rock is more than the Sheling Formation source rock in all the wells under this study,that of the Yongji Formation source rock is relatively least.The lower Eocene Shuangyang Formation of the Well Chang 25 in the Xinanpu sag within the Chaluhe Basin remains the main source unit and presently records the highest expulsion of 97.16(oil:25,gas:72.16)mg/g TOC,followed by the Shuangyang Formation source rocks of the wells Chang 8,Chang 7 and WC 1,are 92.57(oil:28.31,gas:64.26)mg/g TOC,67.11(oil:20.94,gas:46.17)mg/g TOC and 32.06(oil:10.04,gas:22.02)mg/g TOC respectively.Moreover,the hydrocarbon expulsion amount of the Shuangyang Formation source rock is more than the Sheling Formation source rock in all the wells under this study that of the Yongji Formation source rock is also relatively least.Presently,there is no oil expelled from the Eocene Yongji Formation source rock into traps of reservoirs of all the four wells considered in this study.The 2D modeled results reveal entire region from NW to SE in both source rocks is mature to produce oil and gas with vitrinite reflectance values averaging 1.02%Ro.The Bo-Tai sag of this source unit has the highest generative capability.The Lower Eocene source rock lies within the oil window but local enters the gas peak phase and the dominant source rock been the mudstone in the Shuangyang Formation.The upper Eocene Sheling Formation source rock is mainly in the oil window.The fluid potential of the Chaluhe Basin shows the trend of high in northwest and low in southeast,which implies high fluid potential near the depressions while the fluid potential towards the southeastern structural high is relatively low.In summary,after the burial,geothermal,maturation,pressure evolution and fluid potential history models were constructed and analyzed,the Chaluhe basin has the right geological conditions necessary for hydrocarbon exploration and exploitation due to good source rock distribution and maturation,and good geological structure for hydrocarbon accumulation.