Controlling Factors On Quality Of Shale Reservoir Of Longmaxi Formation In The Southeastern Sichuan Basin

The formation of organic-rich shale was controlled by sediment transport and deposition processes as well as sedimentary environment conditions.Based on mineralogy as well as different rock fabrics,biotic components and sedimentary textures,eight types of shale lithofacies were identified in the Longmaxi Formation,i.e.,siliceous shale,silty shale,argillaceous shale,calcareous shale,shell-bearing calcareous mudstone,siltstone-fine sandstone,lime lamina mudstone and bentonite.The shale in Longmaxi Formation was not always able to deposit in a suspending form under the low-energy hydrodynamic conditions.A large number of siliceous biological fossils,biological detritus with size grading characteristics,silt laminations,lenticular bedding,ripple cross-bedding and other sedimentray textures indicate that biogenic deposition,storm deposition and bottom current deposition also existed.Trace element analysis suggests that the siliceous shale in the lower part of Longmaxi Formation was formed in a hypoxic/anoxic environment,and rich in organic matters.Then,sea level gradually dropped.Meanwhile,influenced by bottom current,lithofacies types were changed and shown as silty shale and argillaceous shale,while hypoxic/anoxic environment was damaged,resulting in the gradually worsening preservation conditions of organic matter.Thin section examination,X-ray diffraction and scanning electron microscopy(SEM)determination with cathodoluminescence and geochemical analysis were combined to investigate the Longamxi shales of the Sichuan Basin.The clay mineral assemblage consists of illite + mixed-layer illite/smectite(I/S)+ chlorite.This assemblage includes three sources of clay minerals: detrital,authigenic and diagenetic minerals.These analytical methods allowed us to focus on the origin of authigenic quartz in organic-rich shales.The siliceous shales in the Longmaxi Formation contain up to 56% microcrystalline aggregates of quartz,which are more abundant than silt-size detrital quartz and suggest an authigenic origin for the former.Most silica of authigenic quartz originates from biogenic siliceous dissolution and re-crystallization.TOC and quartz content have positive correlation,indicating high quartz content shales enriched in biogenic silica were beneficial to the enrichment of organic matter.The abundance of interparticle(interP)pores,intraparticle(intraP)pores,and organic matter(OM)pores were measured by point-count and manual-trace techniques.Siliceous shale consists of a high OM pore,with order OM pores(64%)> interP pores(31%)> intraP pores(3%)> microfracture(2%).According to the analysis of sedimentray and diagenetic process,and their impacts on shale reservoir,two factors are suggersted to control the shale reservoir quality of the Longmaxi Formation shales in the Sichuan Basin including:(1)good material base;and(2)unique diagenesis.High productivity and hypoxic/anoxic environment controlled the accumulation and preservation of organic matter.Authigenic microcrystalline quartz aggregates filled in primary pore space and destroyed primary interP porosity.However these authigenic quartz can restrain compaction and preserve the internal pore structure as a rigid framework.Migrated OM filled the associated pore network in authigenic microcrystalline quartz aggregates would more likely to generate a better three-dimensional OM pore network through secondary cracking and provides favorable reservoir spaces for shale gas.