断陷湖盆陡坡带油气成藏类型及其分布规律探讨

Discussion on Play Types and Their Distribution Regularities in Steep Slope Zone of Fault-Depressed Lacustrine Basin

断陷湖盆陡坡带油气生、储、盖层的发育与陆相层序中的体系域具有一定的成因联系 ,不同的陆相层序样式可构成不同的生、储、盖层组合。综合考虑陆相层序储集体类型及其特征 ,将断陷湖盆陡坡带油气成藏类型归纳为三大类、八亚类、十种。它们分别是 :岩性上倾尖灭、岩性下倾尖灭、岩性透镜体、岩性相变、地层不整合、地层削截、地层超覆、地层退覆、构造 -岩性复合、构造 -地层复合等油气成藏类型。这些油气成藏类型在断陷湖盆陡坡带陆相层序各个体系域中的分布具不均衡性 ,以水进体系域为最 ,其次为水退体系域和高水位体系域 ,低水位体系域较少。从陆相层序各体系域中成藏类型的空间展布规律来看 ,水进体系域是陆相断陷湖盆陡坡带油气勘探的重要目的层段。

A complete terrigenous sequence in fault depressed lacustrine basin can be divided into four systems tracts: lowstand systems tract (LST), trangressive systems tract (TST), highstand systems tract (HST) and regressive systems tract (RST). In the steep slope zone of fault depressed lacustrine basin, the development of source beds, reservoirs, cover rocks of hydrocarbon has specific and genetic relationship with systems tracts in terrigenous sequence, and different styles of terrigenous sequence can form different assemblages of source beds, reservoirs and cover rocks. In Biyang fault depression the most favorable source rocks are the mudstone of semi deep or deep lake facies in HST (belonging to regional hydrocarbon producing formation), and in next order the mudstone of lake facies in TST and RST (belonging to local hydrocarbon producing formation); The favorable reservoirs are distributed in sandstone body of fluvial fan fan delta, and in slumping gravity flow fan body of delta slope zone. The regional cover rocks are mudstone of deep lake facies in HST, but the mudstone of lake facies in TST and RST can also form local cover. Synthetically considering plenty of geological factors such as: (1) stratigraphic stacking styles and stratigraphic fray out styles; (2) regional hydrocarbon producing formations and cover rocks, and local hydrocarbon producing formation and cover rocks; (3) several important and special sealed surfaces (e.g. the top and bottom of sequence, downlap surface, basin margin boundary fault, etc.), and according to the types and characteristics of reservoirs in terrigenous sequence, the play types in terrigenous sequence of the steep slope zone of fault depressed lacustrine basin can be generalized into three types, eight subtypes and ten kinds. They include lithological up fray out play type, lithological down fray out play type, lithological facies change play type, lithological lens play type, stratigraphic unconformity play type, stratigraphic trncation play type, stratigraphic overlap play type, stratigraphic downlap play type, structural lithological composite play type, structural stratigraphic composite play type. The distribution of play types in every systems tract of terrigenous sequence is not balanced, and the most popular in TST, popular in RST and HST, the least popular in LST. From the spatial distribution regularities of play types in every systems tract of terrigenous sequence, the play types in TST are distributed in the transitional belt from basin borders to basin centers. Their buried depth is moderate. They are relatively close to source area. Thus, TST is a significant target to hydrocarbon exploration in the steep slope zone of fault depressed lacustrine basin.