Shale Lithofacies Characteristics Study Of Wufeng-Longmaxi Formation In Eastern Maginal Part Of Sichuan Basin

There is 3 classifications of shale lithofacies, classified according to sedimentary structure, classified according to mineral composition, and classified according to combined sedimentary structure and mineral composition. In this article, we choose classified the shale lithofacies according to sedimentary structure. Lamination is the most remarkable sedimentary structure in shale. In this article, we use samples of Wufeng-Longmaxi Formation from 3 well/ outcrops( Wuxi 1st well, X/Y well, Xiushan Rongxi outcrop) located in eastern marginal part of Sichuan Basin for microscopic experiment. We observed 3 major shale lithofacies which are massive shale, parallel lamination shale and non-parallel lamination shale, we also observed several shale lithofacies that are less such widely distributed, which are spotted lamination shale, lenticular lamination shale, and graded lamination shale. In thin section we can observed that the massive shale’s lamination is the most diminished, the parallel lamination shale’s lamination is tenuous and straight, whereas the non-parallel lamination shale’s lamination is thick and intersect with a low angle. The major minerals in a lamina are brittle minerals such as quartz, mica, feldspar, calcite, dolomite, The most common mineral in substrate are clay minerals.The 3 major shale lithofacies in Wufeng-Longmaxi Formation have evident differences in mineral composition. In order of massive shale, parallel lamination shale and non-parallel lamination shale, their average quartz content reduce, which is 61%, 50%, 32% respectively, and their average clay content increase, which is 25.8%, 29.1%, 53.2% respectively. Quartz content is positively related with The TOC content, and clay content is negatively related with The TOC content. In terms of mineral composition, the massive shale and parallel lamination shale’s reservoir physical property precede the non-parallel lamination shale’s. scanning electron microscopic(SEM) experiment can be used to observe lamination’s subtle characteristics, especially the mineral’s orientation. The Wufeng-Longmaxi shale in eastern marginal part of Sichuan Basin develop 8 type of micropores which are inter-particle frame micropore, inter-clayplatelet micropore, particle-rim picropore, intra-framboid micropore, intra-clay micropore, diagenetic dissolution micropore, organic-matter micropore, and microfracture. Although different shale lithofacies in WufengLonamxi shale experience similar diagenetic process and evolvement, however the dissimilarity at the initial sedimentary condition make their microporous characteristic may have some differences. Organic-matter micropore and intra-framboid micropore may more developed in massive shale, while non-parallel lamination shale may have more inter-particle frame micropore and inter-clayplatelet micropore.In terms of organic matter’s type, the 3 major shale lithofacies both are sapropel type and humus type. Massive shale have the highest TOC content and non-parallel lamination shale have the lowest TOC content. Data from the 3 well/outcrops shows that massive shale’s TOC is 3.7%, parallel lamination shale’s TOC is 3.4%, nonparallel lamination shale’s TOC is 1.1%. Major element data of Wufeng-Longmaxi shale sample from eastern marginal part of Sichuan Basin show that, the brittle extent of 3 major shale lithofacies is positively related with The TOC content. Trace element data show that water redox condition of the massive shale and the parallel lamination shale is anoxic to dysoxic, however the non-parallel lamination shale’s water redox condition is dysoxic to oxic. non-parallel lamination shale’s hydrodynamic condition is strongest, while massive shale’s and parallel lamination shale’s are weaker. Sea water paleosalinity of all 3 major shale lithofacies are brackish. Ancient water depth of massive shale and parallel lamination shale are deeper than non-parallel lamination shale’s.Massive shale represents a steady sedimentary environment, whether the deposition rate is rapid or not. It can formed in high deposition rate area such as underwater canyon or deeper part of continental shelf, it also can formed in area where water environment is relatively quiet, deposition rate is low, and suspension dominates, the key is do not have any sedimentary interval. The author of this article consider that the occurrence of lamination in shale represents an environment that relate to periodical river flood(periodical suspension). parallel lamination shale represents an environment where such periodical river flood is not that strong, so that the borderline of layer which deposited during flood and layer which deposited during non-flood season is remarkable and straight, and the erosion is relative slight. The lamination of non-parallel lamination shale is thicker than parallel lamination shale’s, it may reflect much more intenser periodical river flood. In this shale lithofacies sediment that carrid by flood obviously reworked by bottom currents, wave, and tide. As the different assemble assemblage of shale lithofacies in the Wuxi 1st well, X/Y well, and the Xiushan Rongxi outcrop, it may relate to their location on the continental shelf. Due to the higher TOC and quartz content, and the more organic matter micropores, massive shale and parallel lamination shale is more important than non-parallel lamination shale in terms of shale gas exploration and exploitation.