| Kaiping Sag is located in ZhuⅡDepression of the Pearl River River Mouth Basin.At present,the degree of exploration is relatively low.There are 14 exploration wells in the sag and they are mainly located in the high part of the structure.None of them have drilled through all the sedimentary strata in the sag,leading to the lack of systematic disclosure of the deep strata of the sag,especially the hydrocarbon source rocks.However,in recent years,new breakthroughs have been made in the oil and gas exploration in Kaiping Sag,which makes the understanding of resource potential and favorable targets contradict with the exploration practice,Systematic evaluation of hydrocarbon source rocks is urgently needed.Therefore,based on a systematic analysis of the regional geological conditions of the Kaiping Sag and a comprehensive review of previous research work on source rocks,this paper focuses on conducting a systematic study of the planar distribution prediction,quality analysis,and hydrocarbon generation and expulsion characteristics of the three sets of source rocks in the lower Wenchang Formation,upper Wenchang Formation and lower Enping Formation in the Sag,and evaluates the potential of oil and gas resources using different reservoir forming systems as units.Based on the current exploration situation of few wells and no wells in the study area,the method of statistical analysis of the ratio of dark mudstone to formation thickness(dark mud to ground ratio)in the exploration wells,combined with constraints on sedimentary facies,was used to obtain the dark mud to ground specific plane coefficient in different sedimentary facies zones.At the same time,combined with methods such as stratum thickness distribution and geological data inversion,the comprehensive prediction of the planar distribution characteristics of hydrocarbon source rocks was conducted,and the results showed that the hydrocarbon source rocks in each sub sag of Kaiping Sag were distributed in a"scattered lump"shape,The thickness of the source rock in the main depression is the largest,up to 1600 m;The organic carbon content of the hydrocarbon source rock is 1.25%～1.82%,belonging to a medium to good hydrocarbon source rock.The organic matter type is mainly Type II,with a small number of Type II/III sample points.The thermal evolution degree of the hydrocarbon source rock,Ro(%),is between0.46%～0.98%,and is generally in the low mature to mature stage.The biomarkers of hydrocarbon source rocks from the Wenchang Formation and the Enping Formation show significant differences,with differences in spectral characteristics,sedimentary environment,source input,and maturity.The hydrocarbon source rocks from the Wenchang Formation were deposited in a weak reduction-weak oxidation lacustrine environment,and are now mature;The hydrocarbon source rocks in the lower section of the Enping Formation are only distributed in the main Sag and deposited in an oxidation environment of delta swamp facies.The hydrocarbon generation and expulsion characteristics of source rocks evaluated by the hydrocarbon generation potential method indicate that the dark mudstone in the upper and lower Wenchang Formation in the Kaiping Sag generated hydrocarbon earlier,making a greater contribution to hydrocarbon accumulation,while the source rock in the lower Enping Formation contributed less,indicating that the main source rock in the Kaiping Sag is the dark mudstone of the Wenchang Formation.The total hydrocarbon generation and expulsion of Wenchang Formation source rocks are 109.92×10~8t and 37.13×10~8t,respectively,and that of Enping Formation is 17.57×10~8t and 4.41×10~8t.According to the division result of the reservoir forming system and the oil and gas accumulation coefficient of the Pearl River Mouth Basin,the evaluated resource potential results show that the number of prospective resources in the No.I hydrocarbon reservoir forming system(southern slope belt)is the largest,with the strongest hydrocarbon supply capacity.Therefore,the southern slope belt can be the primary target for future oil and gas exploration. |
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