Lithofacies Paleogeographic Pattern and Distribution of Favorable Reservoir Zones in the Middle–Late Cambrian Xixiangchi Formation, Sichuan Basin, China

Abstract

We examined the basin-filling patterns, sedimentary facies characteristics, and favorable reservoir distribution of the middle–late Cambrian Xixiangchi Formation in the Sichuan Basin, China, based on outcrop, drill cores, well-log, and seismic data. Our results suggest the following. (1) The Xixiangchi Formation exhibits a “basal onlap and upper truncation” filling pattern and is divided into six members by four onlap and one erosional slope-break belts, with strata generally thicker in the southeast and thinner in the northwest. (2) Multilevel, southeast-dipping onlap slope-break belts controlled the depositional system transition from tidal flat to restricted platform and finally semi-restricted platform from northwest to southeast. (3) The Caledonian paleo-uplift in the Sichuan Basin underwent three primary evolutionary stages during the middle–late Cambrian: first, low sea level and exposed paleo-highs across most of the Sichuan Basin. Second, high sea level and submerged paleo-highs, with marine deposits across most of the Sichuan Basin. Third, a sea-level fall coinciding with the extension of subaerial exposure in the central–northern part of the basin. (4) Both shoal-facies and karst reservoirs occur in the Xixiangchi Formation. Large-scale shoal-facies reservoirs were developed near the first and second onlap slope-break belts, and relatively favorable composite shoal-karst reservoirs were formed near the third and fourth onlap slope-break belts. This study refines previously established reservoir qualitative speculations by providing a geologically constrained prospective-zone prediction based on stratigraphy, facies, reservoir thickness, and selected test data.