鄂南富县地区奥陶系天然气成藏主控因素分析

Analysis on main controlling factors of Ordovician gas accumulationin Fuxian area,southeast Ordos Basin

鄂尔多斯盆地东南部的富县地区近年来在奥陶系海相碳酸盐岩中勘探成效显著,在马家沟组各种类型的储层中发现了多个含气区,但各含气区之间天然气的聚集程度存在明显差异,造成差异的原因尚不明确,限制了地区勘探的战略展开。结合近期勘探成果,在对该地区成藏基本条件进行分析的基础上,对比解剖马家沟组不同气藏成藏条件,综合分析研究区的烃源岩条件、储层类型和分布规律,认为该区发育风化壳和内幕两个成藏组合,风化壳气藏主要发育在厚源优储有效对接的古地貌高部位;内幕气藏主要发育在近源的沟槽附近及断裂发育带。上古生界煤系烃源岩是天然气成藏的基础;本溪组良好的铝土质泥岩盖层是风化壳储层中天然气聚集的保障;前石炭纪有利的古地貌位置是控制马家沟组储层质量的主要因素,古地貌高部位是风化壳储层发育的有利区,内幕白云岩储层在沟槽等低部位局部连片发育。此外,断裂的发育可促进天然气的局部聚集。研究明确了鄂南富县地区奥陶系天然气藏的成藏主控因素,对勘探从核心区主力层向外向深扩展具有指导意义。

In recent years,remarkable exploration results have been achieved in the Ordovician marine carbonate reservoirs in the Fuxian area,located in southeast Ordos Basin.Various kinds of gas accumulations have been discovered in the Majiagou Formation,each with differing scales of enrichment.However,the factors contributing to these differences remain unclear,thereby hindering the strategic deployment of regional exploration.In light of recent exploration advancements,this paper explores the reservoir-forming conditions in this area,comparing the accumulation differences in gas reservoirs of varying test productivity within the Majiagou Formation.Through an integrated study on source rock conditions and reservoir types and distribution,it has been concluded that two accumulation assemblages have been developed in the Fuxian area:the weathering crust and the interior dolomite.The weathering crust gas reservoir is primarily developed in areas where thick source and excellent reservoirs are efficiently matched,with palaeogeomorphology controlling the gas accumulation.Meanwhile,the interior dolomite gas reservoir mostly develops around the trench near the source or the fault-riddled area.A source rock of certain thickness found in the upper Paleozoic coal measure strata forms the base of gas accumulation,while the excellent aluminaceous mudstone cap in the Benxi Formation provides a stable condition for gas accumulation in the weathering crust reservoir.The favorable palaeogeomorphic position of the pre-Carboniferous period is the key factor controlling the reservoir quality of the Majiagou Formation.The high part of the palaeogeomorphology is a favorable area for the development of weathering crust reservoir,whereas the inner dolomite reservoir is locally developed in lower areas such as grooves.Furthermore,faults can effectively boost local gas accumulation.The research has identified the main controlling factors of the Ordovician natural gas reservoir in the area,which has guiding significance for the exploration and expansion from the core area to the outer and deeper layers in this region.