Prediction of Thin Bed Reservoirs Below 1/4 Wavelength Tuning Thickness Using Full Bandwidth Inverted Seismic Impedance

Abstract

Abstract Although conventional 3D reflection seismic data has been invaluable in the exploration and development of oil and gas fields worldwide, the stand-alone technology fundamentally lacks the resolution required to adequately characterize complex, thin-bedded hydrocarbon reservoirs. Limited vertical seismic resolution, implicitly defined at ¼ wavelength tuning thickness, can become particularly problematic when predicting reservoir dimensions, and ultimately for risk and reserve evaluation. Widess (1973) observed that thickness estimation of a "thin-bed", below ¼ wavelength tuning, can be detected (or is encoded) within the amplitude of the composite amplitude, which results from the increasing constructive interference of the top and base reflections as the bed thins. Herein, a methodology is introduced whereby aggregate reservoir sandstone thickness is successfully predicted away from well control via full bandwidth (0 to 25 Hz) inverted seismic impedance. Thin bed reservoir thickness prediction utilizing inverted seismic impedance has been successfully applied in the onshore Tuscaloosa deep gas (7000 m) trend located in Louisiana, USA, where gross reservoir sandstones less then 40 meters thick are below ¼ tuning thickness. Well logs from 120 wells were used to regionally calibrate the 3D seismic cube to subsurface stratigraphy. A precise calibration allows for accurate rock property and seismic stratigraphic analysis, and tuning-wedge-type modeling; the results of which confirm that seismic amplitude response at Tuscaloosa is true to tuning phenomena, rather than rock property / fluid saturation effects. The 3D full bandwidth inverted seismic impedance data were linearly transformed to a sandstone thickness cube (in meters) well below ? tuning thickness thus doubling seismic thickness prediction by at least an order of magnitude. This technique has resulted in accurate gross sandstone thickness estimation, and ultimately improved volumetrics for risk analysis and reserve estimations. Introduction The explorationist is often exploring for oil and gas accumulations that reside in reservoir sandstones that are often less then 20 meters in thickness. This can become problematic when available 3D reflection seismic data are unable to resolve the vertical dimensions of such reservoirs, and can represent a hindrance when attempting to seismically predict gross rock volume for hydrocarbon reserve estimations. Such estimates represent a critical component to the overall derisking of the prospect inventory when short-listing prospective drilling locations. When bed thickness exceeds the resolving power of the seismic data, the explorationist may resort to alternative means such as probabilistic-type statistical methods in extrapolating / interpolating thickness measurements away from wells. This can be particularly challenging say, for example, in underdeveloped fields where well control is sparse and sand depositional environments are laterally heterogenic characteristic of more fluvial-type settings. Thickness prediction in developed fields incidentally can become difficult if reservoirs are structurally complex due to intense faulting, resulting in compartmentalization, and perhaps depositional expansion of reservoir beds. In such cases, correct thickness measurements of thin beds may not be captured do to infrequent lateral sampling between wells. Which ever the case, the inability of the seismic resolution to vertically resolve thin bed reservoirs adds to the quandary and associated risk of aggregate thickness prediction.