AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Relationship Between Diagenesis, Porosity Evolution and Hydrocarbon Charging of Deeply Buried Sandstone Reservoirs in Dongpu Sag, Bohai Bay Basin, East China
(1) College of Geo-Resource and Information, China University of Petroleum, Qingdao, China.
In the process of study of deeply-buried sandstone reservoirs in Dongpu Sag, which is deeper than 3500m, we found that there are several features: (1) The sandstone is tight, its grains of the reservoir are very fine, and porosity is low. (2) The abnormally high porosity belts were developed locally, but its formation mechanism and distribution laws are unclear. (3) The reservoirs containing high oil saturation usually have high porosity. For example, the siltstone at 4153m in well W203-59 contains oil, and its porosity is 19%. Has the early hydrocarbon charging prevented the process of cementation filling, and left high porosity? Or has the oil just filled the spaces where the early cementation missed? In other words, whether hydrocarbon charging can control the diagenesis and porosity evolution of deeply-buried sandstone reservoirs in Dongpu Sag? And how it controlled? Using the data of thin-sections analysis, cathodoluminescence, SEM and fluid inclusions, we studied the characteristics of diagenesis, porosity type and evolution of reservoirs in different depth, and the relationships between hydrocarbon charging and development of effective reservoir. And gained the following conclusions: (1) Early carbonate cementation had impeded diagenesis, and it had eliminated the further packing of detrital grains during compaction, and is good for the reservation of pore spaces. (2) Latter carbonates mainly include ferroan calcite and ankerite. By the organic acid, early carbonates, feldspar and clay minerals provided material bases for latter carbonates, and the dissolved matters will move to some places by acidic fluid and deposit again, and it usually decreased the porosity of the reservoir. (3) In the reservoirs undertaken hydrocarbon charging, corrosion of feldspars and calcites were strong, and it produced large amount of secondary porosities, which made of the major pore spaces of the deeply-buried sandstone reservoirs. (4) The hydrocarbon charging controlled the diagenetic evolution of the reservoir. It had impeded the quartz overgrowth and latter carbonate cementation filling. (5) The hydrocarbon charging influenced the formation and conversion of clay minerals in the reservoirs . For example, only in the reservoirs in which the hydrocarbon saturation is low, the secondary Illites were largely developed.