THE PRIMARY COMPONENTS AND DIAGENETIC EVOLUTION OF THE LONGMAXI SHALE, SICHUAN BASIN, CHINA: IMPLICATIONS FOR SHALE RESERVOIR QUALITY

The primary composition of grain assemblages and subsequent diagenetic modifications are key factors controlling the pore development and preservation in shale gas reservoirs. The primary components and diagenesis of the Longmaxi Shale have been examined by thin section examination, X-ray diffraction analysis and scanning electron microscopy (SEM) observation with cathodoluminescence and geochemical analysis. The study provides insights into the understanding of differences in grain assemblages significantly control on subsequent diagenetic pathways and their influence on development of different pore types and associated porosity. Extrabasinal components in Longmaxi Shale include quartz, albite, K-feldspar, clay minerals, mica, and heavy minerals. Siliceous skeletal debris (radiolarian and sponge spicule), marine organic matter (OM) (phytoplankton, acritarch and benthic algae) and graptolites are the dominant intrabasinal components. Authigenic minerals consist mainly of quartz, pyrite, calcite, dolomite, ankerite and clays. Pore-filling authigenic microcrystalline quartz is the common cement type. We assume that the major silica source was from dissolution-re-deposition process of radiolarians based on thin section examination and major elements analysis. The depositional and migrated OM represent the two basic types. The depositional OM distribution varies from isolated particles to local concentrations. Migrated OM principally fills the mineral pores network and is likely to form OM networks. OM pores are the dominant pore type in the Longmaxi Shale gas system and the abundance of OM pores is strong positively correlated with authigenic quartz and TOC contents (R²=0.77). We suggest the migrated OM filling authigenic microcrystalline quartz aggregates is the principal matrix for OM pore development. Authigenic microcrystalline quartz aggregates filling primary pore space destroy the interparticle porosity during the early diagenetic stage, but they can restrain compaction and preserve the internal pore structure as a rigid framework. Migrated OM filling the associated authigenic microcrystalline quartz aggregates pore network, is more likely to generate a better three-dimensional OM pore network.