Reservoir Geology and Modelling of Carboniferous Coal-bearing marginal Marine and Fluvial Deposits of Eastern Kentucky and Implications for Hydrocarbon Exploration and Development

Abstract

Successful search and exploitation for hydrocarbons form the subsurface demands the acquisition of a thorough understanding of the basin’s geology, attainable by analyzing critically the stratigraphic framework and facies architecture of analog outcrops and mature fields with a dense network of wells. In this study Petrel was used to construct such a conceptual facies model for the Breathitt Group in the Central Appalachian Basin, with the aim of investigating the pattern of depositional environments and how they influenced the vertical and lateral distribution of facies in the foreland basin. The model provides excellent guidelines to the distribution of depositional energies in analogous subsurface progradational coastal-plain and fluvial-deltaic reservoir sequences. The study area is an appropriate analogue used by many oil and gas companies to understand and solve stratigraphic problems in subsurface coal mines and coal-bearing hydrocarbon reservoirs. Cored borehole data from 12 quadrangles in Eastern Kentucky was loaded into Petrel to generate vertical sedimentological logs. The logs were correlated using extensive coal seams and marine flooding surfaces to obtain stratigraphic columns of the strike and dip sections of the basin. 3D stratigraphic surfaces and isochore maps, embedded with pie-charts, were generated to quantify the proportions of lithofacies along every well that wholly penetrates any given isochore, including the determination, prediction and interpretation of lateral and vertical distribution of facies, depositional energies and paleo-environments. Basic principles of sequence stratigraphy were applied to explain the role and interactions of tectonics, subsidence and eustasy in the evolution of the delta system in the foreland basin. Results show that differential subsidence, including active tectonics (orogenesis) and quiescence gave rise to two broad depositional systems. The upper system, which is predominantly composed of immature sediments, was deposited by braided and meandering streams from the tectonically active thrust-front in the southeast toward the northwest in a fluvial-deltaic environment, whilst the lower system is composed of mature sediments whose deposition was mainly influenced by waves/storms and tides from the sea in the northwest that frequently transgressed the subsiding basin during the long periods of tectonic quiescence. The stratigraphy further shows a tendency to increase slope inclination and a series of anticlinal and synclinal structures, mainly in the upper system, which may provide reservoir traps or control the dynamics of fluid flow in the basin. The analysis shows that the fluvial-deltaic system has better reservoirs with a good vertical and lateral connectivity than reservoirs in the lower coastal-plain system, where vertical connectivity and permeability, is hindered by the thin laterally extensive shales between units. Nevertheless, this system could provide a good source region for the generation of hydrocarbons, due to its high content of organic matter and depth of burial, offering optimal pressures and kitchen (enough heat energy) for the generation of hydrocarbons. Facies analysis using isochores and pie-charts may be applied during both frontier exploration and exploitation phase to provide a simple and quick method of reserves estimates based on the predicted geometrical dimensions of reservoirs and non reservoirs. The method yields important petrophyical parameters that can assist reservoir engineers to design simulation models required for optimal well spacing and positioning, well numbers and better enhanced oil recovery (EOR) methods.