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.

Channing Lake Basin, located west and northwest of Channing, in the Texas Panhandle, is of substantial area and presumably includes lake beds of Pliocene and Pleistocene ages within the Rita Blanca Formation, a member of the Ogallala Group. The foci of this study were a micritic ooid layer and a directly overlying stromatolite layer, which crop out in a canyon approximately 10 kilometers west of Channing. Research was conducted primarily using petrographic and scanning electron microscopy. Significant conclusions include: organic matter was preserved in ooids as filaments and nanoscale spheroids, which served to capture ostracode carapaces within ~10% of micritic cortices; and stromatolites were deposited within an evolving alkaline lacustrine environment producing discernible zones. Potential significance includes improving the understanding the role of organic matter in calcium carbonate precipitation, which has plausible applications in medical, industrial, and academic realms.

The Bahamian Archipelago is one of the few locations in the world where ooid formation is actively occurring. Ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14C, δ18O, δ13C, Mg/Ca and Sr/Ca ratios. Ooids were examined under SEM after each step in the incremental analyses to characterize the nature of dissolution. Radiocarbon dating indicates that surface ooids began forming across the Archipelago between 1000 and 2800 yr BP and continue to form today. The ooids have the same pattern of microboring alteration across the region. The surface and outer cortex of the ooids are punctuated with unfilled microborings, whereas the inner cortex contains two morphologies of aragonite cement filling the microborings. The two morphologies of cement form in association with two different species of cyanobacteria, one is Solentia sp. the other is interpreted to be Hyella sp.. The chemistry of ooids from across the region is remarkably similar. δ18O and δ13C values for all samples vary directly, having a slope of approximately 1. The outer cortex has low δ18O and δ13C values of -3.4‰ and 0.2‰ respectively, whereas the δ18O and δ13C values of the inner cortex are high with values of 1.9‰ and 6.8‰ respectively. The presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid, causing it to appear close to equilibrium with seawater. The isotopic variation in δ18O and δ13C within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings. The most exterior portion of the ooid has very high Mg/Ca values and is interpreted as an amorphous calcium carbonate (ACC) coating. There are two other phases in the cortex, both being aragonite. The outer cortex has a higher Mg/Ca ratio and lower Sr/Ca ratio than the inner cortex. This difference in chemistry is a result of the presence of aragonite cement in the inner cortex. Stable isotopic and trace element results coupled with SEM investigations indicate that microbes do not play a role in ooid formation, but instead alter the texture and chemistry of ooids after they have formed. This alteration occurs throughout the entire shoal region. A new model of ooid formation is proposed whereby a veneer of ACC precipitates on an ooid while it is at the sediment-water interface (the active phase). This veneer of ACC later recrystallizes to aragonite needles, possibly nucleating on organic material and a new cortex layer is formed. Observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis, which allows for a better understanding of paleoenvironments hosting ooid formation. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2009-01-27 13:29:42.765

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

Master of Science / Department of Geology / Abdelmoneam Raef / Advancements, applications, and success of time-lapse (4D) seismic monitoring of carbonate reservoirs is limited by these systems’ inherent heterogeneity and low compressibility relative to siliciclastic systems. To contribute to the advancement of 4D seismic monitoring in carbonates, an investigation of amplitude envelope across frequency sub-bands was conducted on a high-resolution 4D seismic data set acquired in fine temporal intervals between a baseline and eight monitor surveys to track CO₂-EOR from 2003-2005 in the Hall-Gurney Field, Kansas. The shallow (approximately 900 m) Plattsburg ‘C Zone’ target reservoir is an oomoldic limestone within the Lansing-Kansas City (LKC) supergroup – deposited as a sequence of high-frequency, stacked cyclothems. The LKC reservoir fluctuates around thin-bed thickness within the well pattern region and is susceptible to amplitude tuning effects, in which CO₂ replacement of initial reservoir fluid generates a complex tuning phenomena with reduction and brightening of amplitude at reservoir thickness above and below thin-bed thickness, respectively. A thorough analysis of horizon snapping criteria and parameters was conducted to understand the sensitivity of these autonomous operations and produce a robust horizon tracking workflow to extend the Baseline Survey horizon data to subsequent Monitor Surveys. This 4D seismic horizon tracking workflow expedited the horizon tracking process across monitor surveys, while following a quantitative, repeatable approach in tracking the LKC and maintaining geologic integrity despite low signal-to-noise ratio (SNR) data and misties between surveys. Analysis of amplitude envelope data across frequency sub-bands (30-80 Hz) following spectral decomposition identified geometric features of multiple LKC shoal bodies at the reservoir interval. In corroboration with prior geologic interpretation, shoal boundaries, zones of overlap between stacked shoals, thickness variation, and lateral changes in lithofacies were delineated in the Baseline Survey, which enhanced detail of these features’ extent beyond capacity offered from well log data. Lineaments dominated by low-frequency anomalies within regions of adjacent shoals’ boundaries suggest thicker zones of potential shoal overlap. Analysis of frequency band-to-band analysis reveals relative thickness variation. Spectral decomposition of the amplitude envelope was analyzed between the Baseline and Monitor Surveys to identify spectral and tuning changes to monitor CO₂ migration. Ambiguity of CO₂ effects on tuning phenomena was observed in zones of known CO₂ fluid replacement. A series of lineaments highlighted by amplitude brightening from the Baseline to Monitor Surveys is observed, which compete with a more spatially extensive effect of subtle amplitude dimming. These lineaments are suggestive of features below tuning thickness, such as stratigraphic structures of shoals, fractures, and/or thin shoal edges, which are highlighted by an increased apparent thickness and onset of tuning from CO₂. Detailed analysis of these 4D seismic data across frequency sub-bands provide enhanced interpretation of shoal geometry, position, and overlap; identification of lateral changes in lithofacies suggestive of barriers and conduits; insight into relative thickness variation; and the ability of CO₂ tuning ambiguity to highlight zones below tuning thickness and improve reservoir characterization. These results suggest improved efficiency of CO₂ -EOR reservoir surveillance in carbonates, with implications to ensure optimal field planning and flood performance for analogous targets.

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

Sedimentary characteristics of grainy non-skeletal tidal flats along windward platform margins have not been described in modern environments and may be misidentified or misclassified in the rock record. This study describes the sedimentology of such an environment to aid in accurate identification and characterization in the ancient. At Shroud Cay, a grain-dominated tidal flat is sheltered from the high energy of the shelf by a ring of cemented Pleistocene and partly indurated Holocene eolianite islands separated by several narrow tidal passes. Depositional texture, environment of deposition and geobody mapping, extensive sediment sampling, and vibracoring have shown that, though the cemented island provide a barrier from the high energy of the shelf, a high degree of tidal energy still occurs behind this barrier as indicated by the overwhelmingly grainy nature of all of the tidal flat sub-environments. Intertidal flats comprise the majority of the tidal flat surface. These flats are characterized by patchy Scytonema mats overlying bioturbated peloid-ooid grainstones to packstones with cemented lithoclasts. Three main tidal channels dissect the tidal flat and allow diurnal flow, one of those tidal channels does not exit the tidal flat but dead-ends behind a cemented Holocene beach dune ridge along the eastern side of Shroud Cay. Peloid-ooid-skeletal grainstone tidal bars and peloid-ooid packstones fill much of the channels. Most of the channels are bordered by low-relief grain-rich packstone levees often capped by red mangroves and algal mats. The interior-most supratidal parts of the flat, often in the lee of the windward Holocene ridge, are covered by a thick (5-~25cm) Scytonema microbial mat underlain by grain-rich ooid-peloid packstones. Ancient grain-dominated carbonate tidal flats and eolianite deposits like Shroud Cay?s are the reservoir rocks in some of today?s largest hydrocarbon fields. We develop a model for the evolution of the grain-dominated tidal flat, document and compare differences between the grain-rich tidal flat and surrounding environments of deposition, and develop a set of criteria for recognition. These criteria can be used to more accurately characterize reservoirs such as the Jurassic Smackover fields, to avoid mis-classification of similar settings, and more effectively produce those reservoirs.