煤层成因类型及对煤系砂砾岩孔隙演化的控制作用——以准噶尔盆地玛湖地区侏罗系八道湾组为例

Genetic type of coal seams and its control on pore evolution of coal-glutenite:Case study of Badaowan Formation in Mahu area,Junggar Basin

为探讨煤系砂砾岩中不同类型煤层的差异地质响应,及对邻近砂砾岩孔隙演化的影响,以野外露头、孢粉组合、井—震剖面、岩心相序、测井响应、扫描电镜、电子探针能谱及三史(埋藏史—有机酸演化史—孔隙演化史)等资料的综合分析为基础,对准噶尔盆地玛湖斜坡区侏罗系八道湾组煤系砂砾岩中发育的早湖侵期广覆式煤层、高(低)位期局限式煤层进行了综合比对,认为在压实减孔最强烈的准同生—早成岩期,早湖侵期广覆式煤层正值煤系腐殖酸排酸高峰,对邻近砂砾岩储层储集渗流性能的影响整体以抑制性为主,高刚性颗粒含量利于孔隙保存。研究结果表明:玛湖地区八道湾组发育早湖侵期广覆式煤层、高(低)位期局限式煤层2种成因类型。早湖侵期广覆式煤层发育于湖侵体系域TST早期,煤质均一,上覆砾质强陆源阻断沉积,与顶板层之间呈相序突变接触。其分布受控于以逆冲断裂Ⅰ型、Ⅱ型为代表的盆缘边界断裂复活及盆地基底的振荡性沉降,主要分布于近湖盆区首次湖泛面附近,测井响应为极高RT、低DEN、低GR;高(低)位期局限式煤层发育于低位体系域LST、高位体系域HST中期,煤质不纯多夹于静水细粒沉积中,与顶板层、底板层之间多呈现为相序渐变接触。其分布具较明显的相控特征,多分布于水动力较弱的扇间/河道间等低能相带,测井响应为较低RT、较高DEN、较高GR。煤系腐殖酸、烃源有机酸形成的长石粒内溶孔、高岭石胶结物、高岭石完全拟颗粒、高岭石部分拟颗粒等成岩产物在赋存产状、元素组分等方面差异明显。高刚性颗粒含量是煤系砂砾岩储层孔隙有效保存的前提条件,高岭石、硅质等溶蚀产物的迁出程度进一步制约着孔隙的有效性。优质储层区带优选应重点关注高刚性颗粒含量区及水下分流河道、河口坝等高水动力沉积相带。

To explore the differences in the different types of coal seam of coal glutenite geological response,and the influence on adjacent glutenite porosity evolution, based on the comprehensive analysis of field outcrop,sporopollen combination, well-seismic profile, core phase sequence, logging response, scanning electron microscopy, electron probe energy spectrum, and burial history-organic evolution history-pore evolution history,this paper discussed the differences between wide coal seam in early TST and limited coal seam in HST & LST developed in coal-glutenite in Badaowan Formation(J_(1)b), Mahu Sag, Junggar Basin. It suggested that the wide coal seam in early TST is at the peak of humic acid discharge. The influence on the percolation performance of adjacent conglomerate reservoir is mainly inhibitory, and the high rigid particle content is conducive to the preservation of pores. The results show that the J_(1)b in Mahu area has two genetic coal-seams: wide coal seam in early TST and Limited coal seam in HST & LST. In the early lacustrine transgression period, wide coal seam in early TST was developed in the early TST of lacustrine transgression system domain. The coal quality was uniform, the overlying gravel was strongly terrigenous blocking deposition, and the roof layer was in sudden phase sequence contact. Its distribution is controlled mainly by the reactivation of basin boundary faults represented by thrust faults type Ⅰ and type Ⅱ and the oscillatory subsidence of basin basement. It is mainly distributed near the first lake flooding surface near the basin area, and the logging response is extremely high RT, low DEN and low GR. Limited coal seam in HST & LST is developed mainly in the middle LST of low system domain and HST of high system domain, and the impurity of coal is mostly sandwiched in the fine sediment of still water,and the contact between the roof layer and the floor layer is in gradual phase sequence. It is distributed in low energy phases such as interfan/interchannel phases with weak hydrodynamic force, and its logging response is low RT, high DEN, and high GR. The diagenetic products, such as dissolved pores in feldspar grains, kaolinite cements, kaolinite complete quasi grains and kaolinite partial quasi grains, formed by humic acid of coal measures and organic acid of hydrocarbon source, are obviously different in occurrence and element composition. High rigid particle content is a prerequisite for effective preservation of pores in coal-measure conglomerate reservoir,and the migration degree of dissolution products such as kaolinite and silica further restricts the effectiveness of pores. The selection of high quality reservoir zones should focus on areas with high rigid particle content and high hydrodynamic sedimentary facies such as underwater distributary channel and estuary bar.