Hydrocarbon Accumulations in Igneous and Metamorphic Reservoirs
Mark W. Shuster and Christopher K. Zahm
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin
Although oil and gas resources in crystalline reservoirs are not ubiquitous, they may comprise an under-explored resource globally. More than 100 fields with recoverable hydrocarbons in basement or volcanic reservoirs are documented in the literature. This estimate of the number of fields is likely a minimum because information on single well fields and fields in regions such as Russia and parts of the Middle East was not readily accessible. The first reported instance of oil recovered from igneous or metamorphic rocks is from an 1881 test of the Matembo Field in Cuba. Exploration and production of these reservoirs continues today with recent discoveries in the West of Shetlands, offshore U.K.. Of the fields and discoveries with reported volumes, we estimate a total recoverable resource of more than six billion barrel of oil equivalent (boe) from crystalline reservoirs with 13 of these fields each having more than 100 mmboe recoverable.
Trap types include geomorphic-origin “buried hills” and volcanic mounds; structural fault blocks, intrusive sills and laccoliths, “stratigraphic traps” of volcanic deposits and rarely, meteor impact structures. Of these, the “buried hills” are most common with some showing later structural modification. Productive reservoirs are largely dependent on the presence of conductive fractures and fault zones with a lesser component of inter-particle and secondary porosity reflecting diagenetic alteration associated with weathering and fluid flow. Top and lateral seals for most of the accumulations are marine or lacustrine shales with a few cases where impermeable volcanic, evaporite or tight limestone deposits have acted as seals. In some cases the sealing rock directly overlaps the crystalline reservoirs but in many cases, a sedimentary reservoir may overlie the crystalline rocks and in turn be overlain by the ultimate top seal (e.g. shale).
Single well production rates from fractured crystalline reservoirs are field-specific and variable, but maximum oil production rates of individual wells can be high, for example, > 15,000 bopd. Similarly, hydrocarbon column heights vary but many of these fields have hydrocarbon columns exceeding 2000 feet. Many of the fields show pressure connectivity between wells, and these fields show tank-like characteristics which suggests that the fracture systems are connected. Average field porosity is typically 1-2% or less in basement accumulations typified by fractures but in volcanics and where diagenetically enhanced, local porosities can be higher. Field-specific porosities degrade with depth which may indicate a reduction in open fracture density or increased precipitation of cement in fractures. Interestingly, the porosity and permeability profiles for “buried hill” hydrocarbon fields are similar to present day basement aquifers that supply water in many parts of the world (e.g. sub-Saharan Africa, India). These basement aquifers typically show deep weathering profiles as a function of prolonged exposure and surficial or near-surficial weathering. These similarities suggest that the basement oil and gas reservoirs had similar origins.
Key controls on basement oil and gas accumulations include proven hydrocarbon charge with adjacency to mature oil or gas kitchens, timing overlap of hydrocarbon expulsion and tectonic deformation/structural reactivation, and pre-conditioning of ‘basement’ by paleo-weathering.
Mark Shuster (Associate Director: Energy Division) is responsible for managing the Bureau of Economic Geology’s energy-related research. Prior to joining the Bureau in 2016, Mark worked for Shell and affiliates for over 30 years in upstream oil and gas roles around the world includingexploration and appraisal projects in Latin America, Australia, the Middle East/North Africa, Southeast Asia, and North America including the Gulf of Mexico and Alaska. Mark received his Bachelor of Science degree in Geology from the University of the Pacific and his PhD in Geology from the University of Wyoming.