Abstract: Trap and Seal Analysis in Carbonate Shelf Margin Stratigraphic Trap Exploration; Example from the Lower Miocene, Zhujiang Formation, Pearl River Mouth Basin, People's Republic of China
A. J. Lomando, P. J. Chimney, J. P. Popek
Carbonate shelf margin stratigraphic traps are a particularly risky play type because the same diagenetic factors responsible for secondary reservoir quality enhancement can be responsible for degrading seal quality and increasing trap risk. The Zhujiang Fm. prograded westward off the Dongsha Massif initially as a ramp which evolved into a prograding and aggrading shelf margin. Seismic interval velocity analysis, modeled with data from a nearby oil-bearing pinnacle reef, indicates a thick porosity zone at the shelf margin, wedging and pinching out eastward. Core data from the pinnacle reef indicated a short-term exposure event was responsible for significant early secondary porosity enhancement. This also may have influenced the adjacent shelf area; enhancing porosity in he shelf margin but reducing seal capacity in the equivalent up-dip shelf sequence. The seal competency of the overlying Hanjiang shales was demonstrated by hydrocarbons sealed in the adjacent pinnacle reef. Seal risk was evaluated by mercury injection capillary pressure analysis in a shelf carbonate well updip from the margin. It was determined that the up-dip shelf seal is sufficient to hold an economically favorable 300-500 ft (90-150 m) hydrocarbon column.
Seismic evidence revealed several large mega-slumped portions of the shelf margin producing scalloped areas extending several kilometers back into the shelf. Subsequent filling of these reentrants could produce lateral sealing by juxtaposing Hanjiang shales against the sides of the remnant Zhujiang carbonate salient The slump scar on the south side of the salient did not down-cut to a significant stratigraphic depth to provide a critical closure to trap a minimum hydrocarbon column to meet economic thresholds.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France