Abstract:

H-1 sturcture of Huanghekou Depression,is on the south of Bonan Lower Uplift, in which the main reservoir is diamictite. By analysing the characteristics of diamictite and the origin of dolostones, we believe that the reservoir space of diamictite in Shahejie Formation is controlled by the paleogeographic background of sediment, the paleohydrogeologic conditions and the degree of dolomitization later. Paleogeographic background and paleohydrogeologic conditions are the basis for controlling the formation of diamictite. The research area is located in the fan delta front wing which is affected by both terrigenous fresh water and semi-closed brackish water. Such an environment is beneficial to the growth of diamictite which is made up of terrigenous clastic and bioclastic beach. Dolomitization is the key factor to reform reservoir quality, for the mole-to-mole replacement of Ca²⁺ by Mg²⁺ and the selective dissolution after partly dolomitization both enlarge the porosity or permeability. However, there is a great difference among the diamictite reservoirs. When the fresh water has a greater influence on the sediment, the carbonate in diamictite is mainly composed of calcite and the diamictite under such circumstances has poor reservoir capacity. On the contrary, when diamictite are greatly influenced by brackish water, dolomite occupies the most of carbonate and the diamictite has better reservoir capacity. Fossil fragments of back scattering images clearly confirmed the metasomatic origin of dolomite. Based on analysis on paleontology, clay mineral and paleogeographic background data, we found that the Mg²⁺ which dolomitization needs is partly from the transformation of illite-montmorillonite mixed-layer to illite. The water environment of sediment for subtropical climate is semi-closed brackish water and such environment is benefit to the deposition of calcium and enrichment of Mg²⁺. Meanwhile, the rich Mg²⁺ can provide material for dolomitization. The research of controlling factors and development law on diamictite reservoir have positive significance on the exploration of deep reservoirs.

Key words: H-1 structure, diamitictite, controlling factors on reservoir, dolomitization, Shahejie Formation