Research On Diagenesis Of Yanchang Formation Of Upper Triassic In The Southern Ordos Basin

The South of Ordos basin is the major oil-producing areas, and Yanchang Formation of Upper Triassic is its major hydrocarbon-bearing strata. But it is characterized by the universal and particularly low permeability. Through systematically researching on diagenesis of Yanchang Formation, it could find the reasons of low permeability and porosity, and fulfill prospect and evaluation of those zones where the high quality reservoirs are developed. That is of great significance in petroleum exploration in the south of Ordos Basin.The typical representative block of the major Yanchang Formation reservoir were chosen as an example, including. Heshui Area in Xifeng oilfield (the Chang 8 reservoir), Panguliang Area in Jing'an Oilfield (the Chang 6 reservoir), Majiashan-Xiaojianzi Area in Jiyuan oilfield (the Chang 4+5 reservoir), Baibao Area(the Chang 3 reservoir), and Feng110 Area in Panlong oilfield (the Chang 2 reservoir). Using the microscope, the scanning electronic microscope, X-ray diffraction, fluid inclusions, cathodoluminescence analysis, and the paleomagnetism analysis etc, it is systematically studied at petrologic characters, pore structure and poroperm characteristics, the diagenetic characteristic, the division of diagenetic stage, diagenetic facies and the diagenetic evolutionary series of Yanchang sandstone of Upper Triassic in the south of Ordos basin as a whole.The main conclusions are as follows:1. The sandstones of Yanchang Formation in the south of Ordos basin mainly composed of lithic feldspathic sandstone and feldspathic sandstone. It has the characteristics of low composition maturity and middling textural maturity entirely. The distribution of laumontite is characterized by the horizontal zoning feature and the vertical discontinuous distribution condition.2. The associations of pore in the Chang 8 reservoir mainly includes remnant intergranular pores, feldspathic intragranular dissolved pores and a few lithic intragranular dissolved pores; the associations of pore in the Chang 6 reservoir mainly includes remnant intergranular pores, intergranular dissolution pores and a few feldspathic intragranular dissolved pores; the associations of pore in the Chang 4+5 reservoir mainly includes feldspathic intragranular dissolved pores, remnant intergranular pores and intergranular dissolution pores; the associations of pore in the Chang 3 reservoir and Chang 2 reservoir mainly includes remnant intergranular pores, intergranular dissolution pores and feldspathic intragranular dissolved pores;3. According to comprehensive analysis, the reservoir is now in the middle diagenetic phase A to B. The Yanchang sandstone is divided into ten diagenetic lithofacies:①chlorite remnant intergranular pore facies,②chlorite remnant intergranular pore - intergranular solution facies,③chlorite remnant intergranular pore - feldspathic solution facies,④feldspathic solution facies,⑤laumontite and feldspathic solution facies,⑥mica intensive compaction,⑦carbonate cement facies,⑧the authigenic chlorite cement facies,⑨laumontite cement facies,⑩)kaolinite cement facies. Sandstones in the frontal five diagenetic facies are often the high quality reservoirs, but sandstones in the latter four diagenetic facies have poor poroperm characteristics, are usually compacted reservoir.