Characteristics of source rocks and its controls on the formation and distribution of gas from Upper Paleozoic in Southwest Ordos Basin

By using abundance, type, maturity and gas generation intensity of organic matter, the comprehensive analysis of Upper Paleozoic source rocks in southwest Ordos Basin has been made to determine the plane distribution characteristics of main gas reservoirs, and the controlling effects of gas source rock on the formation are analyzed, which has been compared with that in the Sulige area. The Upper Paleozoic source rocks are composed of coal and dark mudstone of Shanxi Formation, Taiyuan Formation, of which coal seams are main source rocks. The average thickness of coal seams is 4.7m in the study area, while 14m in Sulige. Source rocks are abundant in organic matters. That is, the mean residual TOC in coal seam of the second member of Shanxi Formation and Taiyuan Formation are 54.94% and 66.96%, respectively and in mudstone are 2.88% and 1.75%, respectively; The carbon isotope of kerogen varies from -24.56‰ to -22.05‰, indicating that the kerogen belongs to type ; Vitrinite reflectance of organic matter is 1.6%-3.2%, with an average of 2.3%, which is at high to over-mature stage. Gas generation intensity is mainly between (8-20)×108m3/km2, while between (11-29)×108m3/km2 in Sulige region. Compared with the condition of gas source rocks of Sulige region, condition in the study area is slightly bad. Condition of source rocks controls the formation and distribution of gas reservoir in the study area. Horizontally, gas-producing wells are mainly distributed in coal seams with thickness more than 4m and gas generation intensity greater than 10×108m3/km2, showing a trend that the better the source rocks, the more the hydrocarbon content. Vertically, it has a good positive correlation among the vertical migration distance of gas migration, the thickness of coal seam and gas generation intensity. That is, the vertical migration distance of gas becomes farther along with greater thickness of coal seam and higher gas generation intensity. ©, 2015, Science Press. All right reserved.