鄂尔多斯盆地西南部上古生界风化壳型铝土岩系发育过程及优势储层分布规律

Bauxite series in the Upper Paleozoic weathering crusts in the southwestern Ordos Basin:Development and distribution of dominant reservoirs

鄂尔多斯盆地西南部陇东探区上古生界二叠系太原组铝土岩系天然气勘探近期取得重大突破,开辟了新的勘探领域。但是目前对于铝土岩系分布及储层发育特征认识不清,制约了勘探进程。基于岩心观察、薄片、测录井及地震等资料,通过分析铝土岩系的物源、沉积环境及形成过程并探讨铝土岩系储层特征及形成机理,建立了铝土岩系形成分布模式及储层发育模式。研究认为:(1)陇东地区铝土岩系物源主要来自下古生界马家沟组碳酸盐岩。(2)铝土岩系发育在海退背景下温热多雨的沉积环境,垂向结构包括漏斗状、透镜状及似层状3种岩性组合类型,不同成因组合类型受岩溶的容积空间和淋滤程度等因素影响。平面上分布受古地貌控制,高地岩溶潜坑区发育漏斗状铝土岩系,垂向厚度大。斜坡区岩溶阶地以连续透镜状铝土岩为主,且横向分布具有一定范围。洼地区发育似层状铝土岩系。(3)岩溶负地形是控制铝土岩系发育的重要因素,泄水通道的存在是控制优势储层发育的关键因素。高地潜坑和斜坡阶地微古地貌单元是理想的铝土岩系优势储层发育区。该研究成果可为盆地内寻找铝土岩系优质储层提供依据,对新类型天然气勘探提供借鉴。

In the Longdong area—a gas exploration area in the southwestern Ordos Basin,breakthroughs have been made in gas exploration in the bauxite series of the Upper Paleozoic Permian Taiyuan Formation,offering a new exploration target.However,the distribution of the bauxite series and reservoir developmental characteristics within remain unclear,hindering exploration progress.Using data from core and thin-section observations,logs,and seismic data,we analyze the provenance,sedimentary environment,and formation process of the bauxite series and explore the characteristics and formation mechanisms of reservoirs within the bauxite series.Based on these effects,we establish the formation and distribution patterns of the bauxite series,as well as reservoir development model.The results are as follows.(1) The bauxite series in the Longdong area primarily originate from carbonate rocks in the Lower Paleozoic Majiagou Formation.(2) The bauxite series is developed in a warm and rainy sedimentary environment under marine regression.In the vertical direction,the rock series consists of funnel-shaped,lenticular,and stratoid lithologic assemblages.The lithologic assemblages of different origins are controlled by factors such as volumetric space of karsts and leaching degree.Regarding the planar distribution,the bauxite series is dictated by paleogeomorphology.For instance,karst-related sinkholes in the highlands house funnel-shaped bauxite series with considerable vertical thickness;karst-related terraces in the slope zone predominantly contain lenticular bauxite series with a broad and continuous lateral distribution,and the depression zones show the development of stratoid bauxite series.(3) The karstrelated negative landforms serve as a key factor controlling the development of bauxite series,while drainage channels are critical to the development of dominant reservoirs.Furthermore,as micro-paleogeomorphic units,including sinkholes in the highlands and terraces in the slope zone,provide favorable conditions for the development of dominant reservoirs within the bauxite series.These findings provide a basis for seeking high-quality reservoirs within the bauxite series in the Ordos Basin and can be used as a reference for exploring new types of natural gas.