The Research On Characteristics Of Ordovician Palaeokarst And Reservoir In Tahe Oilfield

The Ordovician oil reservoir of Tahe oilfield, which was discovered in 1996, had been extensively explored and exploited since 1988 and its proved reserve reached 28684×10~4t by the beginning of 2005. So the urgent task is seeking for new substitutive blocks of reserves and promoting exploration and development. It is significant to research on the Ordovician palaeokasrst and reservoir for the hydrocarbon is mainly accumulated in the fracture-cave contained reservoir.According to the tectonic and geological evolution and reservoir geology of Tahe Oilfield, the paper which Based on stratigraphy, petrology, mineralogy, well-drilling and logging, geochemistry has made a thorough study on palaeokasrstic characteristics of vertical and horizontal distribution, cycles, landform and mode, and the main controlling factors of development of karst reservoir, then drawn important conclusions as follows:1. The Ordovician palaeokarst of Yahe oilfield formed during middle Caledonian and early Hercynian; The palaeokarst in different region was dominant by different periods as its covered strata and tectonic parts are different.2. There were three karst circles formed during early Hercynian and two during the first episode of middle Caledonian, different region had different karst cycles characteristics.3. The main karst model of the first episode of middle Caledonian was normal water table model. However, there were normal water table model, stratum sand fault controling model and fault controling mode in early Hercynian, the main model in different region was different.4. The distribution law in both horizontal and vertical of karst reservoirs in Tahe oilfield was lucubrated and the conclusion was drawn: The karst reservoirs was mainly distributed in the karst slope along the axes of Akekule uplift.5. The control factors of karst reservoir was discussed in detail and the lithology, tectoncs and karst landforms were the prominent control factors.6. Tahe oilfield is divided into 11 regional blocks for valuation and the best available block for exploration are west of blockⅡand blockⅢ, the next are blockⅣandⅤ.