The Discussion Of Control Factor For Ordovician Paleokarst Reservoirs In Northwestern Of Tahe Oilfield

Carbonate rocks (limestone and dolostone) account for approximately 50% of oil and gas production around the world. As the increasing importance of marine carbonate rocks reservoirs, more new correlation theory are developed reciprocally. But the intensity heterogeneity with multiple-porosity systems is the one of difficulties about the research of the reservoirs in carbonate rocks. As the important paleokarst reservoirs of marine carbonate rocks in our country, the research of Tahe oilfield should to be a important addition to the marine carbonate reservoirs , and according to the research of fluid-rock interaction which show the specific significance about the carbonate rocks of the beneath unconformities to make sure the control factor of the reservoirs development.The basic theory of the paleokarst reservoirs development is the dissolution of the meteoric of epigenesis for carbonate. But actuality, a large number of wells that were drilled into non-porous carbonate rocks beneath unconformities, and it raise a new question that which condition can develop superior quality karst pore-cavity- fracture reseviors. Take the analysis-examination technique of microscope thin section, cathodoluminescence, invariable-trace element, C-O-Sr stable isotope, fluid inclusion and correlation date handle and so on to Ordovician carbonate, the northwestern of Tahe oilfield. Base on these, we give a detail discussion about the fluid-rock interaction in different type carbonate rocks and get some conclusion. The main type is limestone in Ordovician carbonate rocks, it is different with the carbonate rocks reservoirs around the world which are mainly developed with superior quality dolomite reservoirs. Because of the whole average sea-level sustain increase of Ordovician, the sea-floor cementation during eodiagenesis to early-diagenesis, the origin intergranular pore basicly which were developed in high-energy environment had been infilled and proceted completely during the burial diagenesis process because carbonate rocks of Ordovician had not exposured into meteoric during early diagenesis, so the matrix of carboante rocks represent the lowest porosity and permeability which did not own the preserve ability, and this characters influce the development and distribution of karst in epigenesis. There are some matrix of carbonate rocks which proximity to the unconformities did not to be subjected to the meteoric, therefore, the fault and some weak part of structure are easiest to develop karst pore-cavity-fracture reseviors. According to the analysis about geochemistry and represent of calcite which was filled in the fissure show that the thermal fluid, straighary water and meteoric of epigenesis mixture fluid deposited into the karst pore-cavity-fracture which was formed base on the epigenesis during the burial process, at the same time, this mixture fluid migrate path was the just the pathway of meteoric flow which blonged to epigenesis, deposited because of sustained bury and enviroment mutation, and sealed the karst pore-cavity- fracture, level down the reservoirs space.