The role of fluvial and tidal currents on coal accumulation in a mixed‐energy deltaic setting: Pinghu Formation, Xihu Depression, East China Sea Shelf Basin

Abstract

ABSTRACTThe Eocene Pinghu Formation in the Xihu Depression of the East China Sea Shelf preserves mixed‐process deltaic deposits and contains a large number of thin coal seams. This study improves the prediction of coal seam occurrences based on facies distribution and stratigraphic architecture models of deltaic deposits, using core and wireline log datasets. Sedimentological analysis reveals four facies associations, which represent delta plain (including distributary channels and interdistributary bays), delta front, prodelta and tidal flat. These facies associations reflect and preserve the interaction of fluvial and marine processes. Delta‐plain and delta‐front deposits record progressively greater tidal influences when traced southward. Well‐log correlations show that coal‐forming mires on the tide‐influenced lower delta plain were relatively favourable for peat accumulation because the stability of the tidal channels led to a stable platform for peat accumulation on the lower delta plain. The temporal and spatial distribution of coal seams is a function of both autogenic and allogenic responses to forcing. Increased probability of frequent changes in subsidence rates and sea‐level in an active tectonic setting and erosion by channels resulted in thin single‐layer coal seams (mostly 0.5 to 1.0 m). Autogenic processes (for example, delta growth and delta lobe switching) played a significant role in the areal distribution, lateral variation in thickness (ranging from 3 to 71 m) and large cumulative thicknesses (up to 71 m) of coal seams. A general vertical decrease in coal seam thickness likely records a cooling palaeoclimate during deposition of the Pinghu Formation. By comparing delta plain processes to favourable environments of peat accumulation in modern systems with favourable mineralogical, chemical and physical conditions, it can be concluded that: (i) relatively few and discontinuous coal seams developed on the tide‐dominated delta plain generated; (ii) laterally discontinuous and ribbon‐shaped coal seams developed in tide‐influenced deltas; whereas (iii) coal seams formed in river‐dominated deltaic environments have better lateral continuity.