A 3-D seismic volume, wireline logs and core data were integrated to determine the spatial distribution of porous reservoirs within the Wolfcampian-Leonardian detrital carbonate slope and basin strata in Glasscock County, Texas. A 3-D seismic amplitude volume was used to construct a seismic facies analysis of the detrital carbonate section, and generated attribute volumes helped identify detrital carbonate depositional trends, as well as establish a potential correlation between thick detrital carbonate intervals and associated amplitude response. Eight lithofacies were identified in core and were subsequently classified into three main facies: debris flow, grain flow/turbidite, and basinal shale. A facies type log was then created, which was used to supervise the creation of facies logs within other wells to ultimately use in the creation of a 3-D facies model. Cross sections through the study area show an increase in bathymetric relief beginning in Wolfcampian time and continuing through the Leonardian. Detrital carbonate deposition increases dramatically during the Leonardian, consisting of large gravity flows deposited basinward in a northwest-southeast linear trend, rapidly thinning basinward. Individual flows are discontinuous and bounded by basinal shale facies. Four seismic facies were identified within the interval of interest using a structurally smoothed attribute volume, while an RMS amplitude attribute volume provided a correlation between high RMS amplitude values and detrital carbonate thickness. A high RMS amplitude value corresponding to the debris flow facies was extracted from the RMS attribute volume in the form of a seismic geobody. Two facies models and one porosity model were generated by using upscaled values from the gamma ray, total porosity, and lithofacies logs, which were applied over areas with the densest well control. Although the facies model populated from upscaled GR values was useful in stratigraphic interpretation, it is determined that the models should be applied over areas with denser well spacing in order to provide a more accurate and geologically viable subsurface model.

The nose of the Eastern shelf of the Midland Basin is a prominent structural and depositional feature present in Glasscock, Sterling, and Howard counties, Texas. This feature has been expressed in many regional maps and mentioned in some literature, but has not otherwise been studied significantly. This study looks at the viability of using an acoustic impedance seismic inversion to interpret the 2nd and 3rd order sequence stratigraphy of the southern portion of the nose of the Eastern shelf along with its shelf to basin transition in Glasscock, Sterling, and Reagan counties during the Wolfcampian (Asselian-Sakmarian) time (Early Permian). The Wolfcamp Formation (Wolfcampian-Leonardian) was subdivided into six units based on regionally mapped shale markers that correlate with 3rd order sequence boundaries. These horizons were mapped throughout the study area utilizing 3D seismic data and well logs. Analysis of seismic amplitude and inversion (acoustic impedance) volumes, along with well logs were then used to create a 2nd and 3rd order sequence stratigraphic framework in the study area. Six 3rd order sequences and two 2nd order sequences were identified in the study area during the Wolfcampian. From this framework a 2nd order sea-level curve was developed. The oldest Wolfcampian 3rd order sequence is marked by sediment bypass of the shelf and slope into the basin during a 3rd order sea level fall. Shelfal deposition resumed during subsequent sequences as sea-level rose and carbonate production resumed. Carbonate production increased during sequences four through six as part of a 2nd order sea-level highstand. During this highstand the nose of the Eastern shelf grew vertically increasing the gradient of the slope from less than 1° to 3.5°. The end of Wolfcampian deposition is marked by a large number of gravity flows into the basin resulting from subaerial exposure and erosion after a second order sea-level fall.

The Late Pennsylvanian was a time of frequent, rapid glacioeustatic sea-level changes. These changes were recorded in the Wolfcamp D Formation of the Midland Basin as a series of cyclothems similar to those studied in the Midcontinent region (e.g., Algeo and Heckel, 2008). This study focuses on identifying the mechanisms and controls on carbonate deposition and diagenesis through the Upper Pennsylvanian Wolfcamp D Formation and evaluating the potential for these layers to be stratigraphically significant. A stepwise progression of diagenetic processes was identified through the use of δ13Ccarb and δ18Ocarb, bulk geochemical and petrographic analysis, and scanning electron microscopy. Carbonate deposition and early-burial diagenesis appears to be strongly influenced by frequent changes in sea-level and benthic redox conditions. The transition to deep-burial diagenesis was controlled by the thermal gradient in the basin and the extent of diagenesis by the amount of clays and organic matter in the surrounding mudrocks. Further diagenesis was induced through interactions with a brine following clay diagenesis. The presence of multiple phases of diagenesis in this system further highlights the need for several lines of inquiry when evaluating the post-depositional evolution of carbonates in a mud-rich setting.

The Late Paleozoic Ice Age represents a dynamic period in Earth system history recording a shift from icehouse to greenhouse conditions. Concomitant with this change was a series high-frequency, high-amplitude sea level fluctuations leading to the deposition of “Kansas type” cyclothems in the Mid-Continent, and a similar rhythmic expression of interbedded shales and carbonates in the Midland Basin. Stable isotope geochemistry is a particularly powerful tool when examining these mud-rich successions because changes in organic matter partitioning may be recorded first in δ13CDIC of sea-water and consequently in δ13Ccarb of marine rocks. The δ13Ccarb record may also illuminate early and late-stage diagenetic processes and associated destruction of organic matter. This study used high-resolution x-ray fluorescence (XRF) and δ13Ccarb measurements to analyze paleo-environmental conditions in two cores within the Midland Basin Wolfcamp Formation. Using this approach, the combination of more positive δ13Ccarb measurements and enrichment of redox-sensitive elements reflect conditions more favorable for organic matter enrichment in the distal core. Additionally, four (< 6 in.) early-diagenetic intervals have been identified representing prolonged periods of reduced or non-deposition based on sharp negative δ13Ccarb excursions. These horizons are proposed as potential chronostratigraphic tie-points between the two core localities.

Carbonate reservoirs may have a variety of porosity types created by depositional, diagenetic, and fracture processes. This leads to the formation of complex pore systems, and in turn creates heterogeneities in reservoir performance and quality. In carbonate reservoirs affected by diagenesis and fracturing, porosity and peremeability can be independent of depositional facies or formation boundaries; consequently, conventional reservoir characterization methods are unreliable for predicting reservoir flow characteristics. This thesis provides an integrated petrographic, stratigraphic, and petrophysical study of the 'Canyon Reef' reservoir, a Pennsylvanian phylloid algal mound complex in the Horseshoe atoll. Core descriptions on three full-diameter cores led to the identification of 5 distinct depositional facies based on fundamental rock properties and biota. Fifty-four thin sections taken from the core were described are pores were classified using the Humbolt modification of the Ahr porosity classification. In order to rank reservoir quality, flow units were established on the basis of combined porosity and permeability values from core analysis. A cut off criterion for porosity and permeability was established to separate good and poor flow units. Ultimately cross sections were created to show the spatial distribution of flow units in the field.