Swami, Vivek (CGG) | Tavares, Julio (CGG) | Pandey, Vishnu (CGG) | Nekrasova, Tatyana (CGG) | Cook, Dan (Bravo Natural Resources) | Moncayo, Jose (Bravo Natural Resources) | Yale, David (Yale Geomechanics Consulting)
In this study, a state-of-the-art seismic driven 3D geological model was built and calibrated to a petrophysical and geomechanical analysis, 1D-MEM (Mechanical Earth Model), on chosen wells within the Arkoma Basin of Oklahoma. The well information utilized in this study included basic wireline logs and core analysis, including XRD (X-Ray diffraction) data. The traditional petrophysical analysis was augmented with advanced rock physics and statistical techniques to generate the necessary logs. Hydrostatic, overburden and pore pressures were calculated with a petrophysical evaluation model. The 1D-MEMs were based on the Eaton/Olson/Blanton approach with the HTI (Horizontal Transverse Anisotropy) assumption. The 1D-MEMs were calibrated to laboratory data (triaxial tests) and field observations (mud logs, wellbore failure, frac pressures). Therefore, a very good confidence was achieved on Biot's coefficient, tectonic components, anisotropy and dynamic to static conversion factors for Young's Modulus and Poisson's Ratio. Seismic inversions were performed in different time windows and merged to generate high resolution P- and S-Impedance attributes from surface down to the target interval after careful AVO compliant gather preconditioning. A density volume estimate was calibrated to well data, accounting for different geological formations, to decouple P- and S-Wave components as a 3D volume, as well as dynamic Young's modulus (E) and Poisson's ratio (PR). Dynamic E and PR were converted to static parameters using results from 1D-MEMs; and 3D models of Biot's coefficient (α) and tectonic components were built to compute 3D fracture pressure volumes calibrated to well data. The final products were seismic-driven 3D pore pressure and fracture pressure calibrated to 1D-MEMs. The correlation between measured/estimated well logs and corresponding seismic-derived pseudo logs was more than 80%, which indicates good quality of seismic inversion results and hence 3D-MEM. Also, stress barriers, anisotropy, and brittleness indices were calculated on well scale which would help to identify best zones to place hydraulic fractures. The 3D geological model will aid in identifying sweet-spots and optimizing hydraulic fractures.
The restricted, offshore shales of the Mowry Formation (Powder River Basin, Wyoming, USA) have historically been characterised as a key source rock for several Late Cretaceous plays in the region. It is now emerging as an unconventional play where understanding the depositional architecture of the formation is key to predicting the distribution of more prospective areas.
Mowry Formation cores from 6 wells were analysed using sedimentological and biostratigraphic techniques, forming the basis of stratigraphic and facies models. These were used in the generation and calibration of a petrophysical electrofacies model from a dataset of 14 wells, enabling facies interpretation to be extended into the uncored intervals of the Mowry Formation in the study wells. Subsequent facies mapping highlighted particular variations in hypothesised bottom-water oxygenation and quartz sand input. These maps were then combined with 3D seismic attribute, published geochemical and automated mineralogical data, to identify the distribution of depositional facies as a means of improving understanding of reservoir potential.
Close trends between facies distribution, seismic attributes, average TOC, kerogen types and brittle mineral phases were discovered. Regions with lower bottom water oxygenation and lower sand content exhibited lower proportions of brittle phases, higher TOC and a greater proportion of Type II kerogen. Regions of higher bottom water oxygenation and higher sand input typically exhibited greater proportions of brittle phases, lower TOC and a greater proportion of Type III kerogen. These more oxygenated, high sand input facies form distinct lobes originating from a palaeoshoreline located to the west.
It is hypothesised that fluvial-derived, sub-littoral currents were instrumental in delivering brittle material, oxygen and lower salinity water, into the basin. Although this increased the brittleness of the rocks, OM production was probably restricted due to ‘poisoning’ of the nekton by fresh water. Much of the OM that was produced would be subsequently destroyed by benthonic organisms, supported by the oxygen input. Towards the margins of the plumes the freshwater influence and bottom water oxygenation was less, due to water admixing and oxygen dissipation. This allowed for greater OM production and preservation alongside continued high abundance of brittle phases. As a result these plume margins are more likely to exhibit more promising reservoir parameters.
Advances in engineering techniques have re-stimulated interest in some of North America's great hydrocarbon producing basins. Whilst the basins have been extensively mapped, drilled and discussed, the success of future drilling campaigns lies in the detail of these complex systems. The Powder River Basin (PRB) contains a number of stacked plays, each varying in their capacity and complexity. Intra-play heterogeneities only serve to compound this complexity, with micron scale observations impacting regional scale completion programs.
The strata of the Early to Mid-Cretaceous of the PRB reflect the periodic high- and lowstand episodes of the Cretaceous Western Interior Seaway. During this time, varying volumes of water and sediment infiltrated the basin and admixed with the seaway resulting in a series of interbedded marine and terrestrial-dominated silts, muds and sands. Typically these sediments are well preserved as thick successions of stacked plays. The connectivity of these units has delivered the conventionally prosperous Frontier and Muddy sandstone reservoirs, fed by the adjacent Belle Fourche and Mowry shale source rocks.
As the primary source rock in the Mid-Cretaceous section of the PRB, the Mowry shale has recently been targeted as an unconventional reservoir unit. Supported by advancing technologies, a number of recent drilling campaigns targeting the Mowry have delivered significant encouragement for continued investment in the Mowry as a target formation.
Here, we analyze the evolution of the PRB during the Early to Mid-Cretaceous, focusing on the Mowry, and its representation in a variety of media accessible in the present day – outcrop, core, wireline log and seismic data. Through combining these observations and interpretations, we qualify and quantify key stratigraphic members within the Mowry, their distribution, and the residual impression syn- and post-depositional events have had on their constituents.
With this knowledge in place, we grade areas of greatest prospectivity across the basin and support this ranking with evidence from the various datasets.