Biological Reef Reservoir Prediction Of Permian Changxing Formation In WT Area,East Sichuan

Reef is a kind of carbonate formation deposited under biological action.Due to its good porosity and permeability,reef always acts as a high-quality reservoir in oil and gas field formed by carbonate lithologic trap.Several gas reservoirs have been found in the reef in the Permian Changxing Formation.But it is very difficult to acquire a high-signal-noise-ratio seismic data because of the complex seismic-geological condition in this area.At the same time,the physical properties of reef itself are strongly heterogeneous and the reservoir prediction is not precise enough,so it is very difficult to explore and study the reef reservoir in eastern Sichuan.Therefore,it is necessary to study the recognition pattern and distribution rule of reef reservoir with the help of technology obtained in bioherm exploration in recent years to establish a more detailed distribution range of organic reef reservoir.This paper takes the reef deposited in Permian Changxing formation in WT area of eastern Sichuan as the research object to study the advantaged area and oil-gas possibility of it.Combined with geological,drilling,seismic and other data,mainly achieved the following results:The seismic response characteristics of organic reefs in the area are complex.It is concluded by forward modeling that the ‘bright spot' in seismic profile is the comprehensive response of reservoir thickness and porosity,and the strength of‘bright spot' can largely reflects the quality of reef reservoir at the same time.The characteristics of reef morphology and ancient landform are combined to analyze the relationship between them.It is concluded that the paleogeomorphologic features have a controlling effect on the distribution pattern of reefs in the study area.From north to the south,with the decrease of uplift amplitude of platform margin,the reef gradually evolved from single-row reef to multiple-row reef.Reef in platform mainly distributed along the ancient geomorphologic slope break belt of Wenquanjing-Macaoba.To study the relationship between reef reservoir and hydrocarbon accumulation,the paleotectonic evolution in each key period of bioherm gas reservoir is analyzed.The platform uplift in Wenquanjing-Macaoba,Tieshanpo and the platform margin uplift belt on the east side of the trough are located in the inherited paleogeomorphic high belt.Thus,these are the most advantaged areas for gas migration and accumulation.But the tectonic belt between these two is formed by Yanshan movement and Himalayan movement,it is not a favorable area for gas transportation and accumulation.