| dc.description.abstract | Wells on the eastern side of the Midland Basin near its Eastern Shelf in Glasscock County, Texas, penetrate an Upper Leonardian succession of detrital carbonate, deposited in slope and basinal environments. Hydrocarbon production from this interval in Veterans and St. Lawrence Fields is highly variable - some wells produced oil at economic rates following fracture stimulation, whereas the coeval section in other wells failed to produce oil after fracture stimulation.
The depositional texture and diagenetic attributes of the Upper Leonardian detrital carbonate succession were described from slabbed cores, plain light and cathodoluminescence (CL) petrography, scanning electron microscope (SEM) images, and formation micro-imager (FMI) logs. Depth-by-depth assessment of petrophysical properties were interpreted from conventional wireline logs and laboratory core plug measurements using the multimineral analysis method.
Nine main lithofacies were identified on the basis of depositional texture, constituent composition (skeletal and non-skeletal grains, detrital component, and mineralogy) and diagenetic features. The upper and middle slope lithofacies are: mud-lean fusulinid-crinoid packstone and clast-supported polymict conglomerate deposited in channelized settings. The middle and lower slope deposits are: fusulinid-crinoid packstone, fusulinid wackestone, skeletal wackestone and partially silicified skeletal wackestone to mudstone. Occasional intraclasts in these deposits suggest they were deposited by debris flows. The lower slope facies is matrix-supported conglomerate deposited by debris flow at the toe-of-slope. Carbonate mudstone was deposited on the lower slope and on unchannelized parts of the slope. Shale was deposited in the basin.
The Upper Leonardian detrital carbonate succession in Veterans and St. Lawrence Fields was deposited in carbonate aprons developed along relatively gentle to steep slopes (1-50) that were fed by small submarine canyons that by-passed fine-grained upper slope sediments. The rocks were affected most by burial diagenesis. The effects of burial diagenesis on the formation are mechanical and chemical compaction, resulting in reduced pore sizes and loss of primary interparticle porosity, reduced pore throat diameter, stylolite development, grain penetration, grain deformation, and grain fracturing. The pore system throughout the carbonate units is characterized by intraparticle pores mainly in skeletal grains and interparticle pores which are interconnected solution-enhanced pores between grains created by partial dissolution of skeletal grains.
This reservoir characterization indicates that, in Veterans and St. Lawrence Fields, the failure of some wells to produce oil at economic rates could be attributed to the heterogeneities associated with the stratigraphic framework and spatial distribution of depositional facies of the Upper Leonardian detrital carbonate succession. The potential reservoir rocks developed in the channelized portion of the slope whereas non-reservoir rocks accumulated in the lower slope and on interchannel highs on the upper and middle slope. | en |
