ABSTRACT: Sedimentology and Depositional Environment of the Oligocene-Miocene Gas-Bearing Succession in the Yacheng 13-1 Field, South China Sea, People's Republic of China

C. D. Atkinson, S. Bloch, M. H. Scheihing

The giant Yacheng 13-1 gas field, discovered in 1982, is located in the South China Sea some 70 mi south of Hainan Island. The Oligocene-Miocene reservoir succession accumulated on the shallow-dipping flank of an extensional half-graben basin. The reservoir comprises an at least800-ft (250-m) thick clastic interval that forms part of a transgressive basin fill. Basin sedimentation commenced with localized fan deltas sourced from the underlying Mesozoic granitic-metamorphic basement and fed directly into a lacustrine environment. Paleosols (gleysols, planosols, and possible podzoluvisols) and palynomorphs, together with the textural characteristics of the sediments and the common occurrence of coal, suggest a prevailing humid, but seasonal, paleoclimate and abundant vegeta ion. Marine transgression into the basin occurred abruptly from the southwest. An elongate marine embayment was formed, the geometry of which served to amplify prevailing tidal currents. Marine processes had a pronounced effect upon the fan deltas, which continued to supply sediment to the basin. Tidal current and wave reworking winnowed and removed much of the unstable lithic components from the sediments, resulting in increased maturity. With continued transgression, the basement topography was eventually drowned and the fan delta/shoreface sequences were sealed by shelfal muds and silts. Reservoir quality is controlled by burial diagenesis superimposed on the textural and mineralogical characteristics of the sediments. Poor reservoir quality typifies the immature, poorly sorted, lacus rine, fan-delta deposits, whereas good reservoir quality is characteristic of the texturally more mature, tidally reworked, marine, fan-delta, and shoreface sediments.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990