内蒙古二连盆地中部古河道型铀矿床中烃类流体特征与铀成矿关系

Relationship between Uranium Mineralization and Hydrocarbon Fluids Characteristics in Paleo-channel Uranium Deposits of Central Erlian Basin, Inner Mongolia

通过对二连盆地中部下白垩统赛汉组上段古河道型铀矿床砂岩中的酸解烃特征进行分析,探讨了酸解烃中烃类气体组成特征、成因及来源,并结合该区铀矿分布富集的地质特征初步探讨了该区油气与砂岩型铀成矿关系。结果表明,赛汉组上段古河道砂体中CH_4及C_2^+含量与铀含量呈正比,且在氧化带、氧化还原过渡带和还原带内烃类含量具明显的分带性。酸解烃气体组成特征参数C_1/ΣC、C_1/C_2+、C_2/iC_4、C_2/C_3、iC_4/nC_4、iC_5/nC_5、ln(C_1/C_2)、ln(C_2/C_3)表明,研究区赛汉组上段古河道砂体中的烃类气为有机成因的油型气,烃类气主要处于成熟-高成熟阶段,主要为原油伴生气和原油裂解气。根据研究区烃源岩地球化学特征及生排烃史分析认为,该区下白垩统赛汉组上段古河道型铀矿床中烃类气主要来源于下白垩统阿尔善组(K_1ba)湖相腐泥型或偏腐泥型烃源岩,其次为腾一段(K_1bt^1)低熟-成熟阶段的湖相烃源岩。研究区深大断裂、不整合面、主砂体或裂隙是该区深部油气向上逸散的主要通道,深大断裂及不整合面附近应是该区砂岩型铀矿找矿的重点方向;油气在该区古河道砂岩型铀成矿作用过程中具有吸附作用、还原作用和保矿作用,区内砂岩型铀成矿期次、油气聚散史明显受控于晚白垩世末—古新世期的反转构造作用。

Based on analyzing the characteristics of acidolytical hydrocarbon in sandstones of the paleo-channel uranium deposits in the upper section of Lower Cretaceous Saihan Formation in central Erlian basin,the authors investigated the hydrocarbon composition,origin and source of the hydrocarbons in the acidolytical hydrocarbon.Besides,combined with distribution and enrichment geological characteristics of uranium deposits in this area,the authors preliminarily explored the relationship between sandstone-type uranium mineralization and oil-gas in this area.The results show that,in sand body of the upper section of Saihan Formation,content of CH4 and C2+is positively proportional to uranium content,whereas in the oxidation zone,the redox transition zone and the reduction zone,hydrocarbon content develops with obvious zonation.Gas composition characteristic parameters of acidolytical hydrocarbon,i.e.,C1/ΣC,C1/C2+,C2/iC4,C2/C3,iC4/nC4,iC5/nC5,ln(C1/C2),and ln(C2/C3),show that hydrocarbon gas developed in sand body of the upper section of Saihan Formation is organically formed oil-type gas mainly in the high maturity maturation stage,and is mainly composed of crude oil associated gas and crude oil cracked gas.According to geochemical characteristics of source rock and analysis of hydrocarbon generation and expulsion history,it can be inferred that hydrocarbon gas in paleo-channel uranium deposit developed in upper section of Lower Cretaceous Saihan Formation is mainly generated from Lower Cretaceous Aershan Formation(K1ba)lacustrine sapropel or partial sapropel source rock,followed by 1st section of Tenggeer Formation(K1bt)low mature-mature lacustrine source rock.Main channels for upward escape of deep oil and gas consist of deep faults,unconformity surfaces,main sand bodies or fissures in the study area,in which the deep faults and unconformity surfaces should be the key interest for exploration of sandstone-type uranium deposits in this area.In the process of paleo-channel sandstone-type uranium mineralization in this area,oil and gas play a role in adsorption,reduction and ore-enrichment effect.The mineralization of sandstone-type uranium and oil and gas accumulation evolution are obviously controlled by the inversion developed at the end of Late Cretaceous-Paleocene.