Lacustrine Source Rock Development Mechanism And Its Contribution To Hydrocarbon Accumulation In Zhu ? Sub-basin,Pearl River Mouth Basin

Since the first oilfield was found in the late 1980s,intensive petroleum exploration has resulted in the discovery of a series of oil and gas fields in the following 40 years and now has established the annual oil production capacity of 3×10⁶ m³ oil in the Zhu ? sub-basin,Pearl River Mouth Basin and is proven to be an exciting and promising exploration area for oil and gas.However,all these oil fields are medium or small size,no large oilfield is found which is far from the resource assessment.In this context,ascertaining potential source rocks characteristics and differences,classifying discovered petroleum into source-related groups,understanding petroleum distribution pattern and reasonably predicting petroleum secondary migration pathways have been very useful for optimizing petroleum exploration and reducing exploration risks.In this paper,the Zhu ? sub-basin is the object,petroleum system and pool-forming dynamic is the theoretical guidance,hydrocarbon accumulation process is the main line,by combining forward modeling and backward modeling,static description and dynamic simulation,macroscopic and microscopic characterization,qualitative and quantitative description,on the basis of digesting and absorbing predecessors'research achievements,by integrating geological,geophysical and geochemical,drilling,logging and testing data,starting from the development and controlling factors of hydrocarbon source rock,the source rock depositional models were established,the oil and gas were classified to source related groups,source rock thermal evolution and oil and gas filling history were depicted,secondary migration path was predicted,the enrichment pattern of typical reservoirs were summarized and the hydrocarbon accumulation model was established.The Zhu ? sub-basin developed two sets of potential source rocks,i.e.the Eocene Wenchang Formation?E₂w?and Oligocene Enping Formation?E₃e?.The E₂w shows strong heterogeneity,the organic matter in second and third member of the Wenchang Formation(E₂w_2³)is mainly sourced from lower aquatic organisms.The total organic matter content?TOC?is high and kerogen type is good and it was deposited in the semideep^deep,fresh and anoxic lacustrine environment caused by the rapid subsidence,the main product is oil.The organic matter in first member of the Wenchang Formation?E₂w₁?is mainly came from terrestrial higher plants.The TOC is low and kerogen type is bad which is formed in the shore-shallow lake environment and only has potential to generate some gas.The organic matter source of the E₃e is mainly terrestrial higher plants and the TOC is medium and kerogen type is moderate^poor.It was deposited in the lacustrine and swamp,brackish and dysoxic environment during the declining stage.Source rock depositional models for the E₂w and E₃e were put forward.The E₂w was deposited in under-filling condition,the input of terrestrial higher plants is little.The warm and humid climate environment generated lots of rainfall,intensified the chemical weathering on the surface and provide abundant nutrient elements,therefore lead to higher primary productivity.The static and anoxic bottom water environment is favorable for the preservation of organic matter and the anoxic bottom water condition promoted the remineralization of phosphorus and further enhances the surface productivity.The E₃e was deposited in balancing and over-filling condition,the high sediment supply brought a lot of terrestrial organic matter.The relatively hot climate lead to the evaporation and intensified the physical weathering on the surface.The increase of water salinity is not conducive to the development of freshwater algae,and the unstable and dysoxic environment is not good for the preservation of the rich hydrogen organic matter.The development of E₂w was controlled both by the lake productivity and preservation conditions,whereas the E₃e was mainly controlled by the input of terrestrial higher plants.Source rocks in Zhu ? sub-basin have different evolution history.The E₂w in the Wenchang A depression begun to generate petroleum at the end of the E₃e depositional stage,entered the peak hydrocarbon generating stage at the end of Zhuhai depositional stage and has passed into dry gas phase before the Hanjiang regional caprock was formed.The E₃e has just entered the peak hydrocarbon generating stage at the end of the Hanjiang Formation deposition and now at the late peak hydrocarbon generating stage and wet gas phase.The E₂w in Wenchang B depression is in peak hydrocarbon generating stage and wet gas phase and the E₃e in Wenchang B depression is in the early stage of hydrocarbon generation and early peak hydrocarbon generating stage.Crude oils in the Zhu ? sub-basin were divided in to the E₃e-derived oil and mixed oil.The oil found in Wenchang A depression and surrounding low uplift are classified into the E₃e-derived oil group and was believed to be generated from the E₃e source rocks in the Wenchang A depression.The oil in the remote low uplift has more characters of the E₂w source rock,which indicates that the early generated E₂w oil was displaced by the late generated E₃e oil to the remote low uplift.The oil in Wenchang B depression is mixed oil and generated from both the E₂w and E₃e in Wenchang B depression.The oil in the East Qionghai Uplift is E₃e-derived oil and generated from the E₃e in Wenchang A depression,but in the West Qionghai Uplift the oil is mixed type which received supply from both the Wenchang A and B depression.The reason why there is no typical E₂w-sourced oil is the early maturation of the E₂w in Wenchang A depression and charging of the late generated E₃e-derived oil in Wenchang B depression.Hydrocarbon gas in the Zhu ? sub-basin is all organic thermogenic gas,which is the mixture of oil-type gas and coal-derived gas,the combination of primary cracking gas and secondary cracking gas with vitrinite reflectance between 0.6¹.5%.The non-hydrocarbon gas mainly consists of carbon dioxide and nitrogen.The content of inorganic carbon dioxide is high and comes from the deep fluids in mantle which was transported into the upper reservoir through the South Boundary Fault.Thus,inorganic carbon dioxide is mainly distributed near the South Boundary Fault in plane and its content decrease upward in vertical profile.The content of organic carbon dioxide is low and its distribution is unrelated with faults.The nitrogen is concentrated in the lower section and believed to be generated from the bacterial denitrification.Petroleum accumulation patterns in the depressions and surrounding uplifts are different.In the depression,the petroleum was mainly transported from the source rock to the shallow reservoir by faults and assisted by the lateral migration in the sandstones,which is characterized by closing to source rock and strong filling.The oil in surrounding uplifts are mainly conducted in sandstones and structure ridges,supported by the transportation through faults,which has the characteristic of generation from large area and convergence into several traps.Hydrocarbon in the Zhu ? sub-basin is mainly dominated by late charging?since 10Ma?and the source rocks are still at the peak hydrocarbon generating stage.There are two episodes of hydrocarbon filling in the Zhu ? sub-basin,including the liquid hydrocarbon in the first episode and the gaseous hydrocarbon in the second episode.