鄂尔多斯盆地北部上古生界天然气的地球化学研究

Geochemical Study of the Upper Paleozoic Gas in the Northern Ordos Basin

鄂尔多斯盆地北部上古生界天然气的地球化学参数显示出煤成气的特征为高-较高的甲烷化系数、低-较低的C2+组分含量、低凝析油含量、烃类组分均相对富同位素13C、轻烃(C5-C7)中高环烷烃含量等,其源岩无疑是石炭-二叠系(C2b-P1s)海陆交互相腐殖型母质为主的煤系气源岩。天然气地球化学参数中的甲烷化系数在平面上和纵向上的变化趋势与气层组的热演化程度存在着良好的相关性,反映出上古生界低渗透储层天然气的“广覆式生(供)气、持续运聚成藏”特征,天然气的主要成藏时期与气源岩的大量生、排烃时期(J2-K1)相匹配。局部地区的石千峰组发育晚期(K1抬升后)形成的天然气藏。气源岩母质的非均质性、成熟度的差异、运移与成藏特征等多因素叠加作用是引起天然气地球化学参数的变化在总体上的规律性与局部的复杂性共存的主要原因。

The geochemical parameters of natural gas in the northern Ordos basin show the characteristics of coal-derived gas which has high or much higher methanation parameter, low or lower C2+ component concentration, low condensate oil concentration and the isotope of 13C is abundant. Besides, the content of high cycloparafinic hydrocarbon is equivalent in light hydrocarbon, indicating that the source rock is the gas-bearing coal seams of the cross-facies between marine and continental environment. The changing trend of methanation modelus on the surface or vertical direction, connected with the thermal evolution of gas lava, shows that the gas migration and accumulation is persistent and throughout in the low penetrative deposited lava. The gas accumulation period is in line with the hydrocarbon migration period (J2 - K1). But in the local area, the Shiqianfeng group accumulates gas after K1. The heterogeneity and maturity difference of parent substance, and the migrating and accumulating characteristic, these factors superimposed each other causing the variation of whole geochemical features, and it is complex in local area.