大巴山前陆构造纤维状方解石脉成因机理及其油气地质意义研究

Geofluid migration and accumulation in foreland basin or orogenic belt have close relationship with formation and accumulation of oil/gas. Geofluid in foreland basin or orogenic belt has long been a popular subject attracted researchers from different disciplines of tectonic, mineral deposit and petroleum geology. Much attention have been paid to geofluid formation, migration and its records to oil/gas exploration. Fibrous mineral veins, widely developed in low permeability sedimentary rocks such as mudstone and argillaceous limestone, have been considered as the typomorphic mineral which deposited from paleo-geofluid filled along fractures and driven by tectonic stress both in foreland basin and orogenic belt. A lot of research materials confirmed that fibrous mineral veins together with contained fluid inclusions recorded geofluid excretion in deformation process of the foreland basin or orogenic belt. Numerous fibrous calcite veins contained hydrocarbon inclusions were found in the lower Cambrian and the lower Silurian dark shale rock series in Dabashan Foreland belt during our geology research works in the last several years. This proposal will investigate to the field characteristics and to measure the occurrence as well as research to distribution regularities of fibrous calcite veins in Dabashan Foreland Belt. The macroscopic petrography features of fibrous calcite vein together with fluid inclusions will be confirmed. On the basis to restore the paleo-stress field by statistic analysis to occurrence data of fibrous calcite veins, relationship between formation of fibrous calcite veins and deformation process of the Dabashan Foreland Belt will be discussed and fibrous calcite veins formation tectonic dynamics background will be probed. By analysis to geological and geochemical features of fibrous calcite veins and contained fluid inclusions, this research will reveal the paleo-geofluid properties, composition, origin and formation environmental conditions such as temperature and pressure. By combine with the progress of regional geology, this program will discuss the formation mechanics of fibrous calcite vein and its geological significance to hydrocarbon migration and accumulation in the Dabashan Foreland Belt. This proposal has important significance to understand oil/gas accumulation in the Dabashan Foreland Belt.

前陆构造流体形成、运移和聚集与油气运移和成藏关系十分密切，长期以来頗受人们关注。广泛发育在暗色泥页岩和泥灰岩等低渗透性岩石中的纤维状矿物脉体被认为是异常高压下流体形成、排泄和运移的标型矿物，记录了前陆构造不同演化阶段的流体成因、来源、形成条件和运聚特征，在揭示前陆构造油气流体形成和运聚机理方面应用非常广泛。大巴山前陆构造带下寒武统和下志留统暗色泥（页）岩系中发现了大量含烃包裹体的纤维状方解石脉。本项目将在对其野外地质特征调研基础上，确定其宏观地质产状、分布特征和显微岩相学特征。通过对纤维状方解石脉稀土微量元素、同位素等地球化学特征、有机流体包裹体和在该构造带已发现的古油藏沥青地质和地球化学特征综合分析研究，揭示纤维状方解石脉记录的古流体来源、形成条件和运移特征等，探讨纤维状方解石脉成因机理及其反映的油气地质意义。该项目对研究大巴山前陆构造油气运聚特征具有重要意义。

在挤压构造环境中的富有机质黑色页岩中发育的纤维状方解石脉体，被认为是页岩生烃过程中流体超压作用的结果，并且脉体形成与生烃作用同时发生，因此，将含油气包裹体的或沥青的纤维状方解石脉作为页岩生烃和高压排烃的证据。本项目选取大巴山构造带下寒武统富有机质页岩中的纤维状方解石脉体和宁南盆地清水营组中的纤维状石膏脉体，通过详细的野外地质调查、岩相学观察和地球化学等方法，分析纤维状脉体形成动力学机制、流体来源和运移方式，研究大巴山构造带下寒武统富有机质页岩中纤维状方解石脉体形成与生烃作用的关系。研究结果显示，大巴山构造带纤维状方解石脉体和宁南盆地纤维状石膏脉体在结构上具有相似性，均由中间带和纤维状带组成。纤维状方解石脉体的中间带通常由少量围岩碎片和沥青组成，纤维状石膏脉体中间带一般由块状石膏晶体和大量围岩碎片组成；纤维状方解石脉体和纤维状石膏脉体的纤维状带均由平行对齐的纤维状矿物晶体组成，单个晶体具有光学连续性，指示连续生长。大巴山构造带纤维状方解石脉体和宁南盆地纤维状石膏脉体的开启均是由于流体超压作用，但超压成因不同。大巴山构造带下寒武统页岩中流体超压的建立是由于生烃作用（生气阶段）；宁南盆地清水营组页岩中流体超压是由于青藏高原隆升远程效应造成的构造挤压作用。岩相学观察显示，纤维状矿物晶体生长于脉体-围岩界面处，这个过程中没有明显的破裂，结晶力和流体压力可能共同驱动纤维状晶体垂向延伸，但是目前的研究很难确定这两个因素中哪个是主要的驱动力。因此，下一步研究有必要定量计算纤维状晶体生长过程中的结晶力和流体压力的大小。LA-ICP-MS、碳氧同位素、锶同位素和硫同位素结果显示，纤维状方解石脉体和纤维状石膏脉体的成脉物质均主要来自于围岩中方解石和石膏的溶解-再沉淀，纤维状方解石脉体中存在少量有机碳的混入。.总之，通过该项目研究，明确了大巴山构造带纤维状方解石脉野外分布、岩相学和地球化学特征，揭示了纤维状方解石脉成因机理，探讨了纤维状脉体形成与含油气流体形成和排出关系，完成了立项任务。同时，对新发现的宁南盆地纤维状石膏脉体地质和地球化学特征及其形成机理进行了研究。本项目进行到目前为止，发表论文共计24篇，其中SCI论文15篇；获得省部级二等2项；正在申请发明专利2项；培养博士和硕士研究生共计14名。