Tectono-Sedimentary Events of the Xuan Cheng Basin (Anhui Province, China) From Early Paleozoic to Present: Implication for Shale Gas Exploration

Abstract

The Xuan Cheng basin belongs to the Lower Yangtze fold-and-thrust belt and could be defined as a favorable target for shale gas exploration. Despite poor geological information (few well controls, few fresh outcrops and rather low quality regional 2D seismic lines), we realized an updated geological map from the interpretation of remote sensing images constrained by field work observations. During the Cambrian to Silurian, rifting took place in a transtensional setting dominated by NE-SW dextral faults associated with E-W normal faults. Progressive filling of the basin resulted from deposition of basinal mudstones (Cambrian-Ordovician) to clastic turbiditic fans (early Silurian) capped by shallow deltaic facies (upper Silurian). Despite high maturities, the Cambrian Hetang Fm is the main shale gas objective of the Paleozoic series thanks to thick (400–500 m) high TOC black mudstones. The Devonian is marked by a regional scale compression that led to the inversion of previous rift structures and further deposition of coastal sediments. Carboniferous shallow dolomitic breccias may correspond to the end of the foreland basin filling. During the Permian to early Triassic, a deepening of the basin is characterized by progradation and retrogradation cycles of the carbonate platform. Basal transgressive black mudstones also correspond to shale gas objectives (Gufeng and Dalong Fm) possibly in the wet gas to gas window (maturity < 1.9 VRo%). During the late Triassic-early Jurassic, former NE-SW early Paleozoic faults were inverted in a right-lateral transpressional setting. During the late Jurassic, regional erosion with the absence of folding and thrusting can be interpreted as a thermal uplift. During the early Cretaceous, NE-SW transtension is responsible for the development of pull-apart basins filled by continental facies. We interpret this event to be related to initiation of NW-SE oblique subduction of the Pacific plate. During the late Cretaceous-Paleogene, a regional NW-SE extension is associated with the development of NE-SW normal faults giving the final shape of the Xuan Cheng basin. This event is attributed to the slowing-down of the Pacific subduction and subsequent back-arc extension. No folding/faulting is observed in the Neogene-Quaternary sediments. Regardless of the scarcity of data and structural complexity of the area, the integration of all available information allowed defining the location and operational objectives of an exploration well.

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