AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies, Characteristics, and Depositional Model of the Deepwater Fan Complex in the Zujiang Formation, Baiyun Sag, Pearl River Mouth Basin, South China Sea
(1) BP america Inc., Houston, TX.
(2) College of Geosciences, China University of Petroleum, Beijing, China.
The Pearl River Mouth Basin is a Mesozoic-Cenozoic basin located on the north side of the South China Sea. The basin experienced pre-rifting, rifting, and thermal subsidence from Early Cretaceous to early Middle Miocene. Basin history and its corresponding filling sequences share many similarities to those from typical passive margins.
During Early Miocene, when the Zujiang Formation was deposited, a major fan complex (about 220m thick, 252.5km2) was developed under the control of the paleo Pearl River feeder and a shelf-slope-basin physiography. The main body of the complex is located in the east side the Baiyun Sag, a subunit in the basin.
Approximately 500ft of core from 5 wells was described in detail to understand the facies and depositional elements of the fan complex. Internally, ten facies types are recognized and are grouped into 4 facies associations. Seven major depositional elements are identified and these are channel, levee-overbank, inter-channel, lobe, outer-fan, inner-fan, and deep basin. Of these elements, channel deposits are most prolific and usually consist of medium- to coarse-grained sandy facies, including massive, normal- and/or inversely-graded, planar-stratified, and/or deformed sandstones.
Regional sequence stratigraphic analysis based on integrated core, well log, and seismic data shows that the development of the deepwater complex was initiated by a relative sea-level fall. Sediments shed from the shelf margin deltas feed by the paleo Pearl River were transported further down slope under gravity flows, resulting in a basin floor fan. On seismic, the basins floor fan shows distinct onlap up dip and downlap and pinch out down dip. During the ensuing relative sea-level rise, the system stepped back and a slope fan, with clear onlap onto the slope and down lap onto the underlying basin floor fan, was developed. These two together form the fan complex.
Recent discovery of the LW gas field in the fan complex is the first deepwater discovery in the basin, and the discovery shows huge exploration potential in the deepwater region of the Pearl River Mouth Basin and the north margin of the South China Sea.