Depositional Sequence and Lithology Assemblages of Cambrian Evaporite-Carbonate Paragenesis in Sichuan Basin

Abstract

Recently, high commercial gas flow and abundant gas reserves are discovered in Lower Cambrian Longwangmiao Formation in Sichuan Basin, Southwestern China. However, multiple evaporite-carbonate paragenesis developed in the Cambrian, with complex depositional sequences and lithology assemblages, which is the bottleneck problem that has been holding us back from recognizing the reservoir features and distribution patterns. In our research, using seismic, drilling data and the geological interpreted results of loggings, cores and outcrops, paleostructure, lithofacies palaeogeography and depositional sequence are restored. Meanwile, the spatial styles of lithology assemblages in Cambrian evaporite-carbonate paragenesis are established, and high quality reservoirs are predicted. The results are as follows: 1) Thickness of Cambrian becomes thinner from the southeast to northwest. Strata are missing in Leshan-Longnvsi paleo-uplift and its surrounding areas, forming an “onlapping at the bottom, truncation on the top” filling structure. 2) Depositional sequence of the Cambrian changes from mixed deposition of terrigenous clastics and carbonates deposits in the lower part to carbonate platform in the upper part. The sedimentary systems are made of coastal shelf, mixed shelf, ramp or rimmed platform carbonates and so on. The evaporites are mainly found in Lower Cambrian Longwangmiao Formation, Middle Cambrian Gaotaizi Formation and Upper Cambrian Xixiangchi Formation. Scopes and facies of these three evaporates are distinctly different, which is affected by the West Sichuan Central Yunnan oldland, palaeogeomorphology and paleoclimate. 3) There are mainly four lithology assemblages, including evaporite-dolostone, evaporite-limestone, evaporite-siltstone-mudstone, and evaporite-siltstone-mudstone-dolostone. The spatial combination styles include thin interlayered, medium thickness interlayered, and unequal thickness interlayered. The controlling factors are depositional micro-facies, lagoonal sediments and its depositional sequence, and the scale of evaporitic lakes. 4) Major producing layers are distributed in Longwangmiao and Xixiangchi formations. The rock types of favorable reservoir are mainly grain, crystalline, and micritic dolostone and its distribution mainly are around the sequence boundaries and controlled by the grain beach facies, fractures, dolomitization, and locations of the evaporitic lagoons.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015