两元混合天然气定量研究新方法

A Novel Approach for Quantitative Study on Two-source-mixed Natural Gas Reservoirs

确定端元气混合比例是混源气定量研究的主要内容。以两元混合天然气为例,首先要分析混源成藏的地质条件,以确定可能的气源岩;然后根据源岩生气模拟实验与实测数据,建立端元气与混源气的地球化学模型,确定混源气的混合比例与源岩成熟度;最后将结果用于混源成藏过程分析,提出混源成藏的地球化学机制。鄂尔多斯盆地中部奥陶系风化壳天然气为上古生界煤成气和下古生界油型气混源气。从混源气藏形成的地质条件和地质过程着手,利用天然气中甲、乙烷百分组成和碳同位素组成,建立了混合比例估算模型,并绘制了估算图版。对靖边等气田13个天然气样品进行研究,发现混源气混合比变化很大。除去全为煤成气(陕21井)和全为油型气(陕5井)的两个样品之外,11个样品上古生界煤成气和下古生界油型气比例分布分别为22%～84%和16%～78%。但总体以下古生界油型气为主,平均所占比例63.2%,说明鄂尔多斯盆地中部奥陶系风化壳天然气以下古生界自生油型气为主。

Calculating ratio of source-mixed natural gas is important in studying source-mixed gas reservoir. For a two-source-mixed gas reservoir, the following work must be done ： （ 1 ） analyzing the geological factors of formation of source-mixed gas reservoir, confirming potential source rocks, （2） based on experimental data of ascertained source rocks and measured data of natural gas, finding geochemical modes of different end-gas and source-mixed gas, then using the modes to calculate ratios of source-mixed gas and maturity of source rocks, （ 3 ） using the ratios to analyze the formation process of source-mixed gas reservoir, proposing geochemical trapping mechanism. The natural gas in Ordovician weathering crust reservoir in the Central Portion Gas Field in Ordos basin is source-mixed gas of Upper Paleozoic coal-typed gas with Lower Paleozoic oil-typed gas. Starting with analyzing geological factors and process of the source-mixed reservoir, with geochemical parameters of percentages and carbon isotope compositions of methane and ethane, a calculating model of ratios was established and a calculating graph of ratios was given in this paper. 13 gas samples collected from Jianbian gas field were analyzed using the method. The percentages of Upper Paleozoic coaltyped gas and Lower Paleozoic oil-typed gas are 22～84 percent and 16～78 percent, respectively, according to 11 samples （ samples collected from Well Shan21 and Shan5 were excluded）. Based on the result, we could draw a conclusion that Lower Paleozoic oil-typed gas, with the percentage of 63.2, is dominant in Ordovician weathering crust reservoir in the Central Potion gas field in Ordos basin.