The Gas Reservoir-forming Conditions And Accumulation Rules Of Upper Paleozoic In Ordos Basin

Aiming at the subtle gas reservoirs characteristic of low pore, low permeability, low abundance and large area's distribution of the upper Paleozoic in Ordos basin, in the way of studying typical gas reservoirs, in the clue of the gas migration and on the emphasis of high permeability reservoir generation and accumulation regulation, the paper is based on the existing research data , combined with the new theory and progress of the sedimentology, fluid chemistry, petroleum geology, etc, and analyzes systematically geological condition, accumulation model, high beneficial reservoir formation mechanics, distribution regulation of the lithological reservoirs , and points the valid exploration.On the basis of stratigraphic classification and depositional system study, the high resolution sequence stratigraphy theory and way, initiated by T.A.Cross, is adopted in this paper. According to datum lever jacking cycle , isochronous correlation principle and the data of outcrop, drilling, seismic and logging, the strata from Benxi formation to later Shihezi formation is divided into three long cycles and fourteen middle cycles ,and middle cycles are been divided into many short cycles.Using the advanced analysis technology, such as inclusion measuring temperature, calcite cement's electronic probe and isotope analysis, and the water-core reaction theory, the paper sets up the diagenetic model of coal measures strata, time-temperature model and pore evolutionary model, and provides the basis for seeking the high efficient reservoirs.With the studying of valid hydrocarbon source rock evaluation, oil and gas migration, accumulation, and distribution, the paper puts forward three types of gas accumulation combination: inside source type, near source type and far source type, and explains the main characteristic and distribution models of different gas reservoirs.The main controlling factors about upper Paleozoic gas accumulation in Ordos basin are listed in the following:(1) Structural thermal event mainly control the gas forming of upper Paleozoic. It is the later Mesozoic that formed a clear structural thermal event, and resulted in the course of later Jurassic and early Cretaceous, whose gas generation rate is over 0.2 108m3/km2- Ma.(2) Putting the upper Paleozoic gas proximal migration and accumulation models. Gas field's distribution is controlled by gas generation intensity, and it distributed at the region that gas generation intensity large than 20 108m3/km2 from plane, at the same time, the main gas accumulation horizon is inside or near of Taiyuan formation and Shanxi formation.(3) Deltaic sedimentary environment and sedimentation mainly contribute to oil and gas's accumulation, and the main accumulation regions are distributary channel sandstones of deltaic plain, and underwater distributary channel sandstones of deltaic front.(4) High efficient reservoir is the main place of gas accumulation. Its main controlling factors include ancient landform condition, structural evolution, sedimentary facies, sedimentary characteristic of sandstones, diagenesis, corrosion of volcanics and betterment of fracture. Amongthem, later diagenesis corrosion of consedimental volcanics is the key factor of high efficient reservoir, which is in the major of secondary pore.Basing on the analyzing the main controlling factors of forming gas reservoirs, it is pointed that the large lithological gas reservoirs distribution region of Shanbei slope in the middle of basin, is still the main exploration region at present, the west reverse faults and Tianhuan depression of structural pool are the important exploration region, the strategy reserve region is the North Yimeng uplift and South Weibei uplift.