Forward-Inverse Comprehensive Evaluation And Development Model Of Lacustrine Source Rocks In China Offshore

The oil and gas in China Offshore Cenozoic basins mainly come from lacustrine source rocks.Compared with marine source rocks,lacustrine source rocks are more heterogeneous,so it is difficult to evaluate and study the formation mechanism of source rocks.In this thesis,Liaozhong,Huizhou and Weixinan sags,which are representative hydrocarbon-rich generation sags in the Bohai Sea basin,the Pearl River Mouth Basin and the Beibu Gulf Basin in the China Offshore Cenozoic basin,respectively,are selected as the research objects.And the third member of Shahejie Formation,Wenchang Formation and the second member of Liushagang Formation,which are the oil and gas supply intervals,are taked as the research intervals.The development characteristics of China offshore lacustrine source rocks was comprehensively compared and evaluated by the ways of geochemical and geophysical"inversion"and geobiological"forward".The thesis also analyzed the differences of control factors of source rocks among representative sags,determine their main control factors,and establish the development model of lacustrine source rocks.The main achievements and understandings of this thesis are as follows:（1）Based on the study of organic geochemistry of source rocks,cleared the inorganic geochemical characteristics of lacustrine source rocks in representative hydrocarbon rich sags.（1）There were differences in organic matter abundance of lacustrine source rocks in representative hydrocarbon rich sags,the organic matter abundance of source rocks in the second member of the Liushagang Formation in the Weixinan sag,which was good-high quality;followed by Wenchang Formation in the Huizhou Sag,which was medium-good source rocks;the source rocks of the third member of Shahejie Formation in Liaozhong sag were relatively low,mainly developed medium source rocks and some good source rocks.Good-high quality source rocks were mainly developed in middle deep lake facies,and the abundance of organic matter of source rocks gradually decreased from middle deep lake facies,shore shallow lake facies and delta facies.The organic matter types of source rocks are mainly ₁ and ₂,which are mostly in the low-medium mature evolution stage.（2）The good high-quality source rock sections in representative hydrocarbon-rich sag were characterized by high content of Mn,Fe,Ba and low Zn elements,and the pyrite area with high content of Fe and S^2-was the most favorable area for organic matter enrichment.The contents of Fe and S^2-elements in high quality source rocks are greater than 4.4%and 1.4%respectively,that in good source rocks are 3.7%～4.4%and 0.7%～1.4%respectively.（2）Revealed thickness characteristics of lacustrine source rocks in representative hydrocarbon rich sags are,and determine the seismic reflection characteristics of high-quality source rocks.（1）The thickness characteristics of lacustrine source rocks in the representative hydrocarbon-rich sag were different.The thickness of medium source rocks in the third member of Shahejie Formation in Liaozhong sag was the largest,up to590m;The Wenchang Formation in Huizhou Sag has the thickest good quality source rock,up to 195m;The second member of Liushagang Formation in Weixinan Sag had the thickest good-high quality source rock,up to 417m.（2）Seismic prediction and evaluation of source rocks:the seismic facies of good high quality lacustrine source rocks in the representative hydrocarbon-rich sags were characterized by subparallel-parallel,medium-strong amplitude,low frequency,medium-good continuity,and the seismic attributes were characterized by high RMS,average energy,seismic arc length,maximum intensity,the sum of effective bandwidth and low negative amplitude,and the instantaneous frequency.And it was mainly distributed in the center of the sag.（3）Based on the reconstruction of paleoenvironment,the"forward"standard of source rock geobiology was explored and established,and the semi quantitative evaluation of lake facies source rock geobiology was realized.Medium deep lacustrine facies,shallow deep lacustrine facies,shore shallow lacustrine facies and braided river delta facies represent excellent,better,medium and poor paleoproductivity,and paleoproductivity was concentrated in 661.4-1061.8 g C/（cm²·a）,673.0-975.0g C/（cm²·a）,484.0-711.0g C/（cm²·a）,307.5-467.9g C/（cm²·a）respectively;burial efficiency of 0-10%,10%-20%,20%-30%and>30%respectively correspond to poor,medium,better and better geological conditions.The evaluation results show that paleoenvironmental characteristics and semi-quantitative evaluation of geobiology:There were differences in the geochemical characteristics of lacustrine source rocks among representative hydrocarbon-rich sags.The organic matter rocks in the third member of Shahejie Formation in Liaozhong sag was mainly originated from suspected source coccoid algae and continental channel whip algae.From the center to outside to periphery of the sub-sags,the habitat type was from medium deep lake to shore shallow lake to braided river Delta,and the paleoproductivity decreased in turn,the evaluation of biological parameters changed from excellent to better to medium,and the productivity of the lake basin was high;the paleoclimate was warm and humid,and that was deposited in a light salt water and salt lake water with a reduction environment,so the burial efficiency was high and geological（preservation）conditions was superior.The organic matter of the source rocks of Wenchang Formation in Huizhou Sag mainly origined from suspected source chlorella and green algae,and the habitat type was mainly medium deep lake facies and shore shallow lake facies,which had high paleoproductivity;the paleoclimate was warm and humid,and that was deposited in a fresh lake water with a weak reduction-reduction environment,so the burial efficiency was relatively high（that of some sags was low）and geological（preservation）conditions was superior（that of some sags was middle-low）.The organic matter of the source rock in the second member of Liushagang Formation in Weixinan Sag mainly origined from suspected source coccoid algae and continental channel whip algae baise algae.The habitat type was mainly medium deep lake facies,including shallow-semideep lake habitat type,and the paleoproductivity of the lake basin was high;the paleoclimate was warm and humid,and that was deposited in a light salt-fresh lake water with a weak reduction-reduction environment,so the burial efficiency was high and geological conditions was superior.（4）Revealed the internal relationship between geochemical,geophysical"inversion"and geobiological"forward modeling",and further comprehensively evaluated the lacustrine source rocks of representative hydrocarbon rich sags.（1）The geochemistry,geophysical logging,earthquake and geobiology of source rocks were related to each other.Theoretically,the high organic matter rich interval generally presents the logging characteristics of high acoustic time difference,high natural gamma,high resistivity and high neutron and the characteristics of low-frequency,continuous and strong amplitude seismic facies.It can be inferred that it has excellent biological conditions dominated by medium deep lake facies,abundant planktonic algae and high productivity,and better excellent geological conditions with strong reducibility in bottom water of water.（2）Poor,medium and good quality source rocks were developed in the third member of Shahejie Formation in Liaozhong sag;Wenchang Formation in Huizhou sag mainly developed good-high quality source rocks,and some sags developed poor-medium source rocks;the source rocks of the second member of Liushagang Formation in Weixinan Sag were comprehensively evaluated as good high-quality source rocks.（5）The main controlling factors for the development of main source rocks in representative hydrocarbon-rich sags were summarized and condensed,and three development models were established.The results show that tectonism and paleoclimate were the main factors for the differences in the paleoproductivity and preservation conditions of organic matter in the representative hydrocarbon-rich sags of China offshore Cenozoic basins.The different climate zones of Liaozhong sag,Huizhou sag and Weixinan Sag lead to the lower paleoproductivity of Liaozhong sag than Huizhou sag and Weixinan Sag.The continuous and stable temperature stratification and salinity stratification were the main reasons for the formation of the reducing environment at the bottom of the water body in Liaozhong sag;Wenchang Formation in Huizhou sag was mainly temperature stratification,and the bottom of water body was mainly weak reduction reduction environment;The source rocks in the second member of Liushagang formation in Weixinan Sag were mainly temperature stratified,with weak salinity stratification,and the bottom of the water body was a weak reduction reduction environment.Therefore,the biological conditions of lacustrine source rocks in the third member of Shahejie Formation in Liaozhong sag were medium to excellent,and the geological（preservation）conditions were excellent.Good preservation conditions played a leading role in the formation of good to high-quality source rocks,the development of its source rocks belongs to the preservation model of organic matter;The biological conditions of lacustrine source rocks of Wenchang Formation in Huizhou Sag were better,but the geological（preservation）conditions were poor.High paleoproductivity played a key role in the formation of good quality source rocks,the development of its source rocks belongs to the productivity model of organic matter;The paleoproductivity and geology（preservation）conditions of source rocks in the second member of Liushgang formation in Weixinan Sag were high.The development of good quality source rocks was jointly restricted by organic matter productivity and preservation conditions,that belong to model of organic matter productivity and preservation.