How Depositional Environment, Diagenesis, and Thermal Maturity Affect the Evolution and Significance of Organic and Mineral Pore Systems in Unconventional Oil and Gas Reservoirs: Current Understanding and Future Research

Abstract

Mudrocks act as sources and seals for conventional petroleum systems and are sources and reservoirs for unconventional petroleum resources. Complex lateral facies heterogeneity and stacking patterns are typical of mudrock systems. A spectrum of transport and depositional processes impacts original composition, grain assemblages, texture (grain size, shape, and sorting), and fabric (arrangement of particles). Diagenesis (compaction, dissolution, cementation, and replacement) and thermal decomposition of kerogen resulting in generation, expulsion, and migration of petroleum add complexity to the mudrock system by altering porosity, permeability, wettability, and rock strength. Over the past decade, our understanding of pore systems in unconventional reservoirs and the interplay of depositional, diagenetic, and petroleum generation processes in pore system evolution has been significantly improved. However, much is still unknown as we are unable even to predict pore systems in a given lithofacies and/or depositional setting. Establishing linkages between pore systems, permeability, and multiphase fluid flow is an active research effort. Imaging techniques provide a direct means to quantify the proportion and distribution of pore types and the controls on pore development and size distribution. On the other hand, bulk measurement techniques such as gas adsorption, nuclear magnetic resonance (NMR) spectroscopy, small‐angle neutron scattering (SANS), and mercury injection for capillary pressure (MICP) measurement rely on assumptions and models to infer pore dimension and connectivity. Our work utilizes both imaging and measuring techniques and aims to (1) provide a process‐based understanding of the origin and evolution of different pore types and to (2) establish linkages between depositional processes, mineralogy, and variations in pore systems. Examination of Pliocene‐Pleistocene eastern Mediterranean sapropels using field‐emission SEM imaging allowed inspection of petrographic textures before alteration related to thermal maturation. Results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss during early burial. The dominant organic matter (OM) behaved in a highly ductile manner, pervading into some of the interparticle mineral pore spaces, whereas other mineral pores were not pervaded by ductile OM. These primary interparticle and intraparticle mineral pores determine the mineral pore network before petroleum generation and constrain petroleum migration and redistribution during thermal maturation of OM. Therefore, depositional and early diagenetic processes control the subsequent mineral‐pore and OM‐pore network. After OM maturation, pore evolution is closely tied to the thermal cracking and kinetics of different OM types. Laboratory gold‐tube pyrolysis experiments on the Devonian Woodford and Mississippian Barnett siliceous mudstones show that algal cysts (Tasmanites) have better generation and expulsion potential than does amorphous organic matter (AOM). After expulsion, larger pores were found in algal cysts than in AOM. Generated petroleum is chemically fractionated with the expelled products enriched in hydrocarbons and gases and retained bitumen enriched in heteroatomic polar compounds and asphaltenes. Fractionation processes in the mudrock lead to different morphologies of observed pores at each generation stage. With increasing thermal maturation, predominant pore types changed from primary mineral pores, to mineral pores containing relic OM, to coexisting mineral pores and various OM pore types, and finally to OM spongy pores. Residual kerogen and retained solid bitumen both host OM pores in the oil window. In the dry gas window, a solid bitumen/pyrobitumen network, which hosts abundant OM pores, becomes important. We have observed these differences in both gold‐tube pyrolysis residues and by comparing OM pores in naturally matured samples of oil‐window Eagle Ford Formation and dry‐gas window Marcellus Shale. Mineralogy is commonly used to assess rock strength and response to hydraulic fracturing; however, variations of texture and fabric within each lithofacies could be as important as bulk composition. Our work on the Triassic lacustrine Yanchang Formation of the Ordos Basin in China found that mineral pores were more abundant in well‐sorted and coarser grained mudrocks than those in poorly‐sorted and finer grained mudrocks. Lithofacies affects relative abundance of mineral pores versus OM pores in the oil‐window. Lithofacies and TOC both affect permeability in dry gas‐window mudrocks. Primary mineralogy, texture, and fabric determine the following diagenetic processes such as compaction and cementation, and our work suggests that these factors could indirectly affect OM pore sizes, resulting in differences of up to at least two orders of magnitude. There remains a lack of clear understanding on the large‐scale depositional and diagenetic control of pore systems. This is in part as a result of limited and biased sampling in many studies. For example, the OM‐lean facies in unconventional reservoirs are always less studied than the oil or gas‐producing OM‐ rich facies. Also, representative elemental area (REA) of SEM images that can be used to construct 2D or 3D simulations has not been fully established. Cleary, more research is needed to upscale the nm‐ to µm‐ size pore systems to responses in wireline logs and basin‐scale stratigraphic variation. Finally, phase diagrams and fluid dynamics are found to be pore‐size dependence. A multidisciplinary approach integrating geology, geochemistry, petrophysics, and fluid flow is desperately needed to eventually lead to predictability.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019