Abstract

Wells on the eastern side of the Midland Basin near the Eastern Shelf in Glasscock County, Texas, penetrate an Upper Leonardian shallowing-upward succession of detrital carbonates, deposited in upper slope, middle slope, and lower slope/basinal environments. Hydrocarbon production from this interval in the Veterans oil field is highly variable - some wells produce oil at economic rates following cross-linked gel fracture stimulation while others do not produce any oil.

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. We also interpreted the spatial distribution of petrophysical parameters from conventional wireline logs and laboratory core plug measurements. The well logs used include: gamma ray (GR), electrical resistivity, bulk density, neutron porosity, photoelectric factor (PEF) and compressional-wave slowness. The dominant clay type in these units is illite, whereas the non-clay matrix component consists of calcite, dolomite, silt and secondary quartz (chalcedony). The formation fluids are saline connate water and oil.

We identified nine main lithofacies on the basis of depositional texture, constituent composition (skeletal and non-skeletal grains, detrital component, and mineralogy) and diagenetic features. The upper slope lithofacies are: mud-lean fusulinid-crinoid packstone and clast-supported polymict conglomerate deposited in channelized settings.The middle slope deposits are: fusulinid-crinoid packstone, fusulinid wackestone, skeletal wackestone and partially silicified skeletal wackestone to mudstone. Occasional intraclasts in the deposits suggest they are debris flows. The lower slope deposit is matrix-supported conglomerate which is a debris flow deposited on the toe-of-slope. Carbonate mudstone was deposited on the lower slope and unchannelized parts of the slope. The basinal deposit is the shale.

The detrital carbonate rocks in the Upper Leonardian succession were affected by burial compaction. The matrix in all the facies is a mixture of mud and crushed skeletal grains, and most of the larger skeletal grains also are either deformed or broken. The textural effects of compaction on the succession is reduction of pore-size and loss of porosity, stylolite development, grain penetration, grain deformation, and grain fracturing. The pore system throughout the carbonate unit is characterized by intraparticle pores in skeletal grains, and interparticle pores which are interconnected solution-enhanced pores between grains. Calcite is the most abundant cement, occurring as poorly- to well-developed bladed fringes in the intraparticle pores, syntaxial overgrowth of crinoids, and subhedral to anhedral blocky calcite in both the intraparticle and interparticle pores. Baroque dolomite occurs in intraparticle and interparticle pores filling dissolution vugs. Silica (chalcedony) replacement in grains also is common.

This reservoir characterization indicates that the average initial water saturation of the Upper Leonardian detrital carbonate succession in the Veterans Field decreases from south to north, and that the clast-supported polymict conglomerate has the highest combined values of porosity, permeability and oil saturation, and therefore represents the best quality reservoir rock.