AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Controlling Factor of Reefs in the Upper Triassic Maantang Formation and Their Impact on Hydrocarbon Exploration in the Western Sichuan Basin, China
(1) Research Institute of Petroleum Exploration & Development-Northwest, Petrochina, Lanzhou, China.
(2) State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China.
Until recently, the upper Triassic Maantang Fm. has been examined in detail by many geologists, though patch reef complexes are host to some of the world’s most prolific and complicated Hydrocarbon reservoirs. This is also the latest marine carbonate layer in the Sichuan basin. The investigations have been made on 22 patch reefs in it. Through measured sections, systematic sampling and analysis of the thin sections, it is concluded the Carnian reefs were mainly constructed by the siliceous sponges, which based on the homoclinal carbonate ramp from 30m to 200m below the sea level.
The reefs are from 2m to 70m high. The reefal subfacies complexes can be easily distinguished, including the reef base, the reef core, the reef flank, the interreef and the reef top. The interreef is bedded and its thickness is obviously thinner than the reef core and the reef flank. The reef top is composed of black bedded mudstone. The outcropping area of the reefs is along the Longmenshan Belt and the length of the area is ca. 66 km along NE direction, and is 1-2km in width.
The development and extinction of them may indicate the evolution process of the Sichuan basin and the adjacent area. The reef base developed in shallow water and high energy without terrigenous material. Then the increasing terrigenous material was deposited and the water became deeper. The surroundings were gradually filled by siliciclastic sediments so that the reefs died away. Consequently the silty mudstone in the Xiaotangzi Fm. was deposited. The process could coincide with the tectonic movement of the area.
The porosity & permeability of the samples is typically low. However comparing the serials of the Oxfordian reefs in Europe, according to the evolution of the global siliceous reefs, the colliculus uplift in the seismic profiles and a commercial gas flow in Chuanke 1 well in Maantang, the relative shallow-sea, high-energy reefs may exist near the central Sichuan Basin. They should be a target for the natural gas exploration.