CHARACTERISTICS AND GENESIS OF COMPOUND RESERVOIR SPACE IN TUFFACEOUS GLUTENITE—A CASE FROM SHAHEJIE FORMATION ON THE SOUTH SLOPE OF LAIZHOU SAG

A large number of hydrocarbon reservoirs have been discovered in the near source tuffaceous glutenite of Shahejie Formation in the Laizhou Sag of the Bohai Sea. Different from the traditional understandings that near source sand and gravel deposits are usually poor in physical properties, the tuffaceous glutenite is rich in hydrocarbon accumulation under the control of the composition and sedimentation of the tuffaceous matter. Complex reservoir spaces are well developed. By means of cores, thin sections, field emission scanning electron microscopy, cathodoluminescence and mercury injection, 4 types of reservoir space are identified in the tuffaceous glutenite of Paleogene, i.e. the inherited, the diagenetic, the structural and the mixed. We proposed and defined for the first time in this paper the concept of glutenite compound reservoir. The glutenite reservoir, as observed, is commonly superimposed by two or more types of reservoir spaces under the control of various geological factors. Inherited reservoir spaces, which includes leaching holes, devitrification holes, structural fractures, clay shrinkage cracks, primary fractures, and mechanical fractures formed during transportation, are often changed by later reformation. Inherited reservoir spaces account for about 41% of the total, which play a critical role in the glutenite reservoir. Diagenetic and structural reservoir spaces are closely related to the development degree of inherited reservoir spaces. If the inherited reservoir space exists, the late diagenetic fluid and tectonic movement may further increase the reservoir space and the quality of glutenite reservoirs will be much improved. Therefore, the genesis of sandstone conglomerate reservoirs may be classified into 4 types i.e. reservoirs formation by weathering and deposition, reservoirs formation by alteration of tuffaceous matter, the change in supporting matter and later dissolution. A sedimentary and diagenetic evolutionary model is then established upon the above study of the compound reservoir space.