四川盆地元坝地区须家河组泥质烃源岩超压水力破裂动力学机制与排烃通道模式

Primary migration is the bottleneck for the understanding of hydrocarbon accumulation. It has been generally accepted that hydrocarbon is mainly expulsed by overpressure-driven in independent hydrocarbon phase through micro-fractures. Since 1990s, the dynamics of hydrocarbon expulsion has made significant progress, but kinematics studies (e.g. channel, direction, transport capacity, etc. ) have been difficult to advance. The key reason lied in "rarely leaving traces from hydrocarbon expulsion", making primary migration research neither have ideal clue nor find evidence. The previous studies of my group showed that bitumen-filled hydraulic fractures were widely developed in the source rock of Xujiahe Formation (upper Triassic) in Yuanba area, Sichuan Basin (Figure 1). Both the bitumen and hydraulic fractures were confirmed to be the traces of hydrocarbon-expelling, so it provided an ideal clue to study primary migration. Through reconstructions of paleo-pressure、paleo-tectonic stress using fluid inclusion microthermometry and PVT thermodynamic modeling、acoustic emission experiment, respectively, we will decipher the main stress and overpressure regimes of source rock during hydrocarbon generation window, and then predict the hydraulic fracture development and distribution pattern based on rock-failure theory. On the other hand, we will establish the bitumen-filled fracture distribution model through macroscopic and microscopic of core observation. Finally, we will use the established model to verify the predicted pattern, revealing the mechanism of hydrocarbon expulsion and establish the mode of hydrocarbon expulsion conduit of the source rock in the interested area.

油气初次运移是油气成藏机理研究的瓶颈，现今普遍接受的排烃机制是：在超压驱动下油气以独立烃相通过微裂缝排出。90年代以来，源岩排烃动力学取得了重要进展，但运动学研究(如通道、方向、运移量等)却举步维艰，关键问题在于源岩排烃很少留下痕迹，使得初次运移缺乏有效的研究途径，也找不到证据印证理论研究成果。本项目组前期研究表明，四川盆地元坝地区须家河组泥质烃源岩普遍发育沥青充填水力破裂缝(见图1)，确认这些沥青及其充填的水力破裂缝是初次运移留下的痕迹，为初次运移研究提供了理想的"天然实验室"。通过流体包裹体显微测试恢复古压力和声发射实验恢复古构造应力，确定烃源岩主生烃期应力与超压状态，应用岩石破裂理论预测水力破裂缝分布模式；另一方面，通过岩心宏观与微观观测，建立沥青充填水力破裂缝的分布模式，据此验证预测的水力破裂缝分布模式，从排烃动力与通道耦合层次，揭示源岩排烃机制，建立排烃通道模式。

基于须家河组烃源岩发育分布、生烃潜力、生烃历史，储层致密化机理与致密砂岩发育分布，以及超压发育演化研究，阐明了元坝－通南巴地区须家河组多尺度、多层次、彼此封隔的压力系统；泥质烃源岩与砂砾岩储层裂缝中广泛发育固体沥青，这些固体沥青是古稠油裂解的产物，因古原油粘稠度高而残留于排驱路径中，成为排烃通道的标志，由此建立了烃源岩排烃通道模式。该区烃源岩排烃受构造应力－生烃增压联控诱导超压的驱动，排烃通道主要由顺层及穿层的超压水力破裂缝组成。烃源岩排烃可以分为时空上连续的两个过程，即初始“凝聚”和其后经微裂缝的排放。初始凝聚过程烃类相态及通道与动力学机制还知之甚少，但其后微裂缝排放通道的形成主要受构造应力与超压的控制，超压水力破裂缝与构造裂缝是烃源岩排烃的重要通道。