Permian Prograding Carbonate Margin without Marginal Reef and Implications for Hydrocarbon Exploration: A Case Study from Eastern Precaspian, Kazakhstan

Yang, Rongjun; Zhang, Yajun; Li, Guobin; Hao, Tao
PetroChina Research Institute of Petroleum Exploitation & Development-Northwest, Lanzhou, China.

The prograding carbonate bank margins are not fully researched though several similar margins have discovered in the Miocene sediments of the Gulf of Papua, Maldives, and Grand Bahamas. At the same time, the genesis, composition and evolution of them are yet not very clear. Now a broad prograding complex without reef is revealed in the eastern Precaspian, Kazakhstan in the ancient stratigraphy (the lower Permian). Through comparing among them, it is favorable to understand the Permian prograding margin.

The data from three-dimensional seismic and several wells are used for interpretation during the research process. Then one prograding complex is examined in details in the northern margin of the carbonate platform while some marginal reefs developed in the other part of the platform. The complex looks like an oval with a north-south long axis in the isopach map. And it is thick in the centre and thin around the rim. Due to the following erosion and multistage tectonic compression, the top of complex is absent and some Characteristics have been changed. However the important features can still be identified during the research.

Each prograding sequence can be subdivided in two packages: strong- and weak-amplitude reflection packages. Strong-amplitude reflection packages consistently display a basinward shift in onlap and/or downstepping and are interpreted as having formed during falling relative sea level. Weak-amplitude reflection packages are interpreted as having formed during the rise and highstand of relative sea level. Each sequence thus represents a complete sea level cycle. The evolution process is in accordance with the marginal reefs in the other part of the platform.

Meanwhile the two wells have penetrated the rim of the complex (weak-amplitude reflection packages), the composition is mostly mudstone. But no well reaches the strong-amplitude reflection packages which are considered to have relatively coarse grains, higher grain-to-mud ratio than the rocks which are responded through weak-amplitude reflection packages. So it is suggested that the exploration target for hydrocarbon exploration may focus on the area with strong-amplitude reflection packages rather than the slope-break zone of the carbonate platform as conventional thinking.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012