THE DEPOSITIONAL CHARACTERISTICS AND OIL POTENTIAL OF PALEO PEARL RIVER DELTA SYSTEMS IN THE PEARL RIVER MOUTH BASIN, SOUTH CHINA SEA

Delta systems in the Zhuhai, Zhujiang and Hanjiang formations are inte rpreted as having formed during periods of sea-level rise from the lat e Oligocene to mid-Miocene (30-10.5 My B.P.). Deltas in the Zhuhai For mation were developed in a shallow water environment on a sandy, wave- or fluvial-dominated coasts. Thick and blanket-like sandstones are co mmon but mudstones are infrequent. Oil-bearing zones have been found o nly in the upper part of the systems, overlain by mudstone of the Zhuj iang Formation. Deltas in the Zhujiang Formation are interpreted as ha ving formed in a deeper water environment on sandy, wave- or fluvial-d ominated coasts. Carbonate platforms started growing on the Dongsha ma ssif as the sea-level slowly rose. The moderate sandstone/mudstone rat io in the delta systems and a zone of secondary porosity in the carbon ate rocks provide an excellent reservoir rock and seal rock for the ba sin. Deltas in the Lower Hanjiang Formation are similar to those of th e Zhujiang Formation, but the upper part of the Formation is interpret ed as having formed in a shelf environment; the thick shelf mudstone, interbedded with offshore bar sandstones, would be one of the better r egional seal rocks in the study area. No carbonate rock developed on t he Dongsha massif due to the rapid rise in sea-level. All delta system s from the Zhuhai to the Zhujiang Formation are stacked in an onlap pa ttern indicating a more and more expanding marine transgression, which was caused by eustatic sea-level rise. This marine transgression is a lmost unique and contrasts to the progradational (offlap) pattern of C enozoic delta systems in the Gulf of Mexico. It resulted in thick sand stone deposits, immature or poorly mature source rock and no ductile m udstones, with no growth faults or rollover structures. Hydrocarbons g enerated from Eocene-Oligocene lacustrine source rocks are trapped by drape structures related to local basement highs or by carbonate rock with secondary porosity. Most of the remaining undiscovered reserves i n the basin will be found in non-structural, subtle traps, especially stratigraphic traps.