A petrophysical study of the upper grainstone/packstone reservoir of the Oxfordian Smackover Formation in Little Cedar Creek Field was conducted, integrating core description, thin section analysis, log interpretation and cathodoluminescense to characterize controls on oil production in the upper reservoir. Little Cedar Creek Field produces approximately 2.4 million barrels (bbls) of oil annually and is currently in secondary recovery. By analyzing petrophysical characteristics such as porosity and pore type and correlating them to facies changes, better predictions can be made to optimize secondary recovery. The diagenetic history of the ooid-peloid grainstone records six separate events. Early marine phreatic dogtooth sparry rim cement helped create the framework that allowed it to maintain a good portion of its depositional porosity as it underwent subsequent compaction, dissolution and cementation events. The most common porosity types are vuggy, oomoldic and intercrystalline. The Smackover Formation ooid-peloid grainstone/packstone unit consists of multiple alternating ooid-peloid grainstone and peloid packstone/wackestone facies with varying porosity types. The most common types are oomoldic and vuggy with a range of preserved intergranular porosity. Porosity in the grainstone facies averages 17% and 5.6% in the packstone/wackestone facies. The number of facies changes within the upper reservoir does not play a significant role in controlling well production. Facies changes are too thin to be identifiable utilizing well logs alone, although neutron and density well logs do trace a close relationship between log values and core plug analysis values of porosity. Core reports indicate that porosity and permeability correlate strongly with pore size and facies. Areas with thicker accumulations of grainstone facies have higher porosity and permeability values and have higher oil production. Isopach maps of the cumulative grainstone facies indicate thick build-ups parallel to strike for the formation, consistent with a shoal environment. The strongest predictor of well production is the cumulative thickness of grainstone facies within the grainstone/packstone unit of the Smackover Formation. The grainstone is thickest in the southwest part of the field and pinches out updip in the northwest. Secondary recovery gas injection would be most effective if applied in the southwestern portion of the field because it could effectively sweep the oil updip towards the stratigraphic trap.

Ooid grainstone reservoir architecture remains poorly understood, particularly because of sedimentologic and stratigraphic heterogeneities that are innate to grainstone body development. Understanding of Geospatial relationships and recovery of hydrocarbons from these significant reservoir facies can be improved with access to outcrop analog information from well exposed examples. Object-based models and other modern subsurface reservoir models are considered superior methods for portraying realistic sediment distributions. These models, however, are highly dependent on input data describing sediment-body geometry for faithful template generation. Such input data are notably rare in carbonate systems. Maps generated from modern depositional patterns give a first approximation of areal distribution, but they are not as useful for understanding final preserved stratigraphic thickness and internal facies, sedimentary structure, and grain-type patterns. For this purpose, studies of exceptional outcrops are required. The 18 km long oblique-dip-oriented wall of the Shattuck Escarpment provides such a unique exposure of Permian-age grainstones. The Shattuck Escarpment in the Guadalupe Mountains provides an oblique-dip profile that exposes a near-complete middle Permian Grayburg mixed clastic-carbonate shelf succession of three high-frequency sequences which contain 30 high-frequency cycles. Particularly important for this study are the four cycles that display full updip to downdip extents of ooid grainstone tidal bar and tidal delta objects. The data from the Shattuck wall presented in this paper focusses on the transgressive portion of the upper Grayburg, or G12 high-frequency sequence (HFS), located 5 km landward of the time-equivalent shelf margin. This interval is an analog for productive fields along the northwest shelf of the Delaware Basin and on the eastern flank of the Central Basin Platform. The goal of this project is to understand the sedimentology and facies/cycle architectural variability of tidally influenced shelf crest ooid grainstones of the Grayburg Formation. Comparing this outcrop data to modern grainstone deposits allows the reader to understand the small-scale and large-scale sedimentologic and architectural patterns in analogous subsurface ooid grainstone reservoirs. Spatial analysis of these cycles was carried out using measured sections and GigaPan (high resolution photomosaic) data. Petrophysical (Porosity and Permeability) data was collected from three separate vertical core plug transects approximately 1 km apart with a vertical resolution of 30 cm. Cycle-set-scale grainstone complexes up to 6m thick extend at least 4.25 km along depositional dip and show variations in permeability between 6-400 mD and porosities between 8-20% within the lower portions of the grainstone complex. / text

The Permian (Leonardian) aged Upper Spraberry Formation found in the Happy Field of Garza Co. TX, contains one of the best examples of a reservoir composed of resedimented carbonates in a deep-water slope-basin setting, with numerous whole core of wells with full suites of electric logs, high resolution 3D seismic coverage, and 20+ years worth of production data. Sequence stratrigraphic analysis from seismic data combined the lithologic analysis from outcrop analog, core, and well log data helps identify that the Happy Field is located within the transgressive systems tract of the fifth composite sequence in the Leonardian. The reservoir is composed of discrete allochthonous ooid and skeletal grains transported downslope via hyperconcentrated density flows sourced from a re-entrant in the shelf margin and deposited in a long-lived topographic depression at the toe-of-slope. Vertical heterogeneity due to layers of shaley silt punctuated by successive flows of oolitic and skeletal grains along with lateral heterogeneity created by younger flows of material create reservoir compartmentalization which can impede efficient development. Core-calibrated electric log correlations aids in the mapping of isolated compartments which helps with efficient development planning for the field. / text