Sequence architecture,sedimentary evolution and hydrocarbon exploration potential of the large Cambrian platform margin in Lunnan-Gucheng area of Tarim Basin
In response to the unclear types and temporal-spatial distribution of the large-scale reservoir-cap assemblages of Cambrian platform margin in Lunnan-Gucheng area of Tarim Basin,based on the latest collected/merged three-dimensional seismic data,more than 30 two-dimensional seismic survey lines,thin sections of rock debris from 9 drilling wells,and C/O stable isotope curves from the Lunnan-Fuman area,the paper establishes a chronostratigraphic framework from platform interior to platform margin to basin,carries out the research of sedimentary facies using the sequence or system tract as a unit,makes clear large-scale reservoir-cap as-semblages and source rock distribution,and evaluates favorable exploration zones.The research results show as follows.(1)Four second-order sequences and 11 third-order sequences are identified in Cambrian platform margin.Horizontally,there are significant differences in sequence architecture.The Lunnan area features a complete structure with typical lateral stacking,while Gucheng area partially lacks sequences from Terreneuvian to Epoch2(SQ1-SQ3)and has the prominent characteristic of vertical accretion and stac-king.(2)Lunnan-Gucheng area experienced four evolution stages in succession,i.e.,mud-rich ramp,low-angle progradation ramp to weak-rimmed platform,rimmed platform dominated by vertical accretion evaporation lagoon,and lateral accretion flat-topped rimmed platform,and the spatial differentiation was controlled by Tanan oldland and ancient sea level.(3)Nine sets of platform mar-gin mound-shoal belts(SQ3-SQ11),3 sets of gypsum salt rock belts(SQ5-SQ7),and 6 sets of source rocks(SQ1,SQ6,SQ8-SQ11)have been developed in Lunnan-Gucheng area.(4)Four types of effective reservoir-cap assemblages,i.e.,the platform margin mound-shoal belt covered with gypsum,the residue mound-shoal belt covered with soil,the mound-shoal belt covered with compact carbonate rocks,and the slope gravity flow,were evaluated and five types of hydrocarbon accumulation models were established.The platform margin mound-shoal belt covered with gypsum and the residue mound-shoal belt covered with soil are the best reser-voir-cap assemblage,where recent breakthroughs have been made and the depth of drilling can reach 10 000 m.The gravity flow on mud slopes is also a new type of reservoir-cap assemblage worthy of exploring.
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