Sequence Stratigraphic Framework and Coal Accumulation of the Lopingian Longtan Formation in the Sichuan Basin, South China

Abstract

ABSTRACT The Lopingian Longtan Formation in the Sichuan Basin is a representative coal‐bearing succession deposited in a marine–continental transitional setting. Pronounced lateral variability in depositional environments and the subtle expression of sequence‐stratigraphic surfaces have, however, led to continued debate over the regional sequence framework and the controls on coal accumulation. This study integrates drilling, well‐log and core data from 146 wells with geological identification of key stratigraphic surfaces and Hurst‐exponent analysis of gamma‐ray logs. The Longtan Formation is divided into three third‐order sequences (SQ1–SQ3) and the lithofacies palaeogeography of each sequence is reconstructed. Depositional environments comprise volcanic residual plains, fluvial–floodplain systems, tidal flats and carbonate platforms. Coal accumulation was concentrated in tidal‐flat peat swamps during SQ1 and SQ2 and declined markedly during SQ3. Coal‐accumulation centres migrated overall from northeast to southwest. Coal seams C19 and C24 in SQ1 and C12 and C14 in SQ2 are the most laterally persistent and stable. Major seams formed mainly during the middle part of the transgressive systems tract and the early part of the highstand systems tract, when relatively slow sea‐level change maintained a close balance between the rates of accommodation creation and peat accumulation. These conditions promoted sustained peat accumulation and the formation of thick coal seams.