Charles Kerans

Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas

Petroleum production from restricted shelf carbonates of the Lower Ordovician Ellenburger Group is commonly considered to have been a result of a pervasive, relatively homogeneous tectonic fracture system within the reservoir rock. However, regional facies and diagenetic (paleokarst) studies of Ellenburger strata, based on cores and wireline logs, have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit. Ellenburger Group carbonates, which attain a thickness of over 1,700 ft (520 m), record sedimentation on a shallow-water restricted shelf occupying most of west Texas. Logging of over 10,000 ft (3,050 m) of core from 63 wells has allowed recognition of six facies assemblages: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone-grainstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod-intraclast packstone-grainstone. These facies assemblages record initial transgression and subsequent progradation and aggradation. Paleoslope was generally south and east from the Texas arch toward the Ouachita and Marathon orogenic belts. All facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, are characterized y very low intergranular and intercrystalline porosity. Porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand prior to transgression of Simpson Group siliciclastics. During this episode, a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolomites (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor. Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, Block 13, and several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized laterally extensive collapse structures, such as in the Shafter Lake reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model.

Integrated Characterization of Carbonate Ramp Reservoirs Using Permian San Andres Formation Outcrop Analogs

The San Andres Formation (Permian, Guadalupian) of the Permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character, complex facies and permeability structure, and generally low recovery efficiencies of 30% of original oil in place. The approach used here to describe carbonate ramp reservoirs such as the San Andres Formation produces detailed reservoir models based on integration of sequence stratigraphic analysis, petrophysical quantification through definition of rock fabric flow units, and fluid flow simulation. Synthesis of these subdisciplines clarifies which aspects of the geologic-petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil satur tion. The San Andres Formation crops out along the Algerita escarpment, a long, oblique-dip, continuous shelf-to-basin exposure in the central Guadalupe Mountains. These outcrops provide a unique opportunity to study lateral relationships in geologic and petrophysical structure analogous to those occurring between wells in subsurface reservoirs. On the basis of sequence stratigraphic analysis, three scales of cyclicity are recognized: depositional sequences, high-frequency sequences, and cycles. Examination of the cycles in two detailed window areas provides a practical scale for petrophysical quantification and fluid flow simulation. An understanding of cycle position within the high-frequency sequence framework also provides predictive information. Petrophysical analysis revealed six rock fabric groups dominated by intergranular, separate vug, or dense intercrystalline pore types. Comparison of these rock fabric groups with facies descriptions produced a rock fabric flow unit model that honors the geologic structure of the cycle and sequence framework. Permeability data were averaged within rock fabric flow units using a geometric mean approach based on fine-scale fluid flow modeling of deterministic and stochastically generated permeability fields. Two-dimensional black oil fluid flow models illustrate that (1) major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell-scale geologic structure as determined by outcrop geologic and petrophysical mapping, (2) an understanding of static geologic/petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture, and (3) because of the orderly distribution of high- and low-permeability facies within cycle stacks of high-frequency sequences, this larger scale of geologic description can give a reasonable first-order approximation of fluid flow patterns and early breakthrough. End_Page 181------------------------------

Marine Diagenesis in Devonian Reef Complexes of the Canning Basin, Western Australia

Marine diagenesis is widely recognized as a critical process in the development of reef complexes, both ancient and modern (James and Choquette 1983, James and Ginsburg 1979, Krebs 1969, Mountjoy and Krebs 1983, Playford 1980, 1984, Purser 1969, Schroeder 1972, Walls and Burrowes 1985). Marine cementation, together with frame-building reef organisms, is largely responsible for building reefs into rigid, wave-resistant structures with positive relief above the seafloor. Indeed, it has been suggested for some reefs that cementation was the dominant reef-building process and that they should therefore be regarded as inorganic “reefs” (Schmidt 1977).

Three-dimensional geological and synthetic seismic model of Early Permian redeposited basinal carbonate deposits, Victorio Canyon, west Texas

The Lower Permian outcrops of Victorio Canyon, in the Sierra Diablo Mountains in west Texas, show undisturbed stratigraphy of carbonate toe-of-slope and basinal deposits. These rocks consist of a vertical stack of carbonate debris-flow deposits and hyperconcentrated density-flow deposits, thick skeletal-ooid grainstone deposited as basin-floor fans, various reworked muddy carbonate deposits, and fine siliciclastic siltstones. This succession was deposited within five sequences that include the upper Hueco Formation (late Wolfcampian) through the Bone Spring Formation (middle Leonardian). Using a ground-based light detecting and ranging-generated high-resolution digital outcrop model (DOM) aged as a template, we mapped and digitized the stratigraphy of the toe-of-slope and basinal deposits on the 5-cm (2-in.) precision DOM. On the basis of the digitized stratigraphic contacts, several regional surfaces were constructed, and a 3-D geocellular model was built. Facies information within this model is extrapolated from measured section data using both a kriging algorithm and stochastic simulations. Using impedance values extracted from a subsurface analog, a 3-D impedance model was created for both the kriged and the stochastic models. Both models incorporate fine-scale stratigraphic architecture. In addition, the stochastic impedance model incorporates spatially correlated noise, resulting in more realistic synthetic seismograms. Three-dimensional synthetic seismograms were calculated at 20, 40, and 80 Hz. The reservoir-prone facies is skeletal-ooid grainstone deposited as a 1.5-km 750-m (0.93-mi 2460-ft) basin-floor fan up to 15 m (49 ft) thick. This basin-floor fan is subtly imaged in vertical seismic section at a frequency below 80 Hz. It is, however, better recognized on time slices with peak frequency as low as 20 Hz and even better delineated on horizon slices that parallel the stratigraphy.

Seismic frequency control on carbonate seismic stratigraphy: A case study of the Kingdom Abo sequence, west Texas

Conceptual models and real three-dimensional (3-D) seismic data show that in progradational carbonate platform margin and slope deposits of the Kingdom Abo reservoir of the Permian basin, west Texas, primary seismic reflection events do not necessarily follow clinoformal geologic-time surfaces. The seismic frequency content of the data controls the dip and architecture of seismic reflection events. High-frequency seismic data tend to follow thinner, time-bounded clinoform depositional elements (time-stratigraphic units), whereas low-frequency seismic data tend to image thicker, low-angle lithofacies units (time-transgressive units). In seismic data of moderate frequency, both clinoform units and flat lithofacies units are imaged, creating complex interference patterns that are difficult to interpret.Experiments with models and real data demonstrate that seismic data can be selectively filtered in the signal bandwidth to help distinguish time-stratigraphic units from lithostratigraphic units. Selective filtering alters the dominant frequency of the data to match a desired scale of geologic objects. If there are enough high-frequency components in the seismic data, true clinoform stratigraphy can be imaged even if the data are dominated by lower frequency components.Seismic modeling of outcrop of the Abo sequence in Apache Canyon, Sierra Diablo, west Texas, indicates that a dominant frequency of 100 Hz is needed to recover true clinoform stratigraphy using seismic data. The interpretation of available 3-D seismic data can only partially distinguish time-stratigraphy from lithostratigraphy because of the lack of frequency components greater than 70 Hz in the data. Application of this outcrop model in seismic modeling avoids interpretational pitfalls that can occur if the dominant frequency of the seismic data is not matched to the unit thicknesses that need to be resolved.

Outcrop-constrained hydrogeological simulations of brine reflux and early dolomitization of the Permian San Andres Formation

Our hydrogeologic model tests the effectiveness of brine reflux as the mechanism for early dolomitization of the Permian San Andres Formation. Brine circulation is constrained by sequence-stratigraphic parameters and a heterogeneous distribution of petrophysical properties based on outcrop data. The model simulates accumulation of the San Andres platform and calculates fluid flow and solute transport in response to relative sea level fluctuations. It tracks porosity loss caused by compaction and the concomitant permeability feedback. The amount of dolomite potentially formed is calculated by means of a magnesium mass balance between brine and rock. Results show that (1) brine reflux is an effective mechanism to deliver magnesium to dolomitize large carbonate successions; (2) relative sea level–controlled transient boundary conditions result in intricate flow and salinity patterns that can generate irregular dolomite bodies with complex spatial distributions; (3) pervasive dolomitization can result from several short-lived reflux events by the amalgamation of brine plumes sourced in different locations and times; and (4) the model successfully recreates the dolostone and limestone patterns observed in San Andres outcrops.

Analysis of hyperspectral and lidar data: Remote optical mineralogy and fracture identification

Karst systems are widely recognized as highly complex and often extremely productive reservoirs of water as well as petroleum. They are also often associated with mineralization. The availability of a large (several tens of square kilometers), well-preserved paleokarst outcrop is rare; therefore, maximizing the information that we can extract from examples like the Franklin Mountains is critical to the study of karst-related fluid flow. The mapping process is confounded by the need to map very large areas to find relatively small and somewhat unpredictable zones of extreme deformation. Moreover, the brecciated regions interpreted to be of karst origin are often composed of the same lithology as the surrounding rock and thus make traditional remote sensing data such as multispectral satellite imagery or photographic data inadequate to delineate such systems. The Franklin Mountains in El Paso, Texas, expose lower Paleozoic carbonates deposited over a giant carbonate platform referred to as the Great Ordovician Bank. The limestone-dominated bank was subsequently modified by surface karst and several large, vertically extensive caves that occupy up to 70,000 m2 of outcrop each. The breccia bodies are preferentially dolomitized within the limestone host rock. The size of these features is ideal for testing dolomite-calcite identification with high-elevation hyperspectral imagery at 20-m × 20-m pixel size. Terrestrial-based lidar (light detection and ranging) data were also utilized to identify collapse brecciation highlighted by hyperspectral image analysis. Results of this study delineate the distribution of dolomite and calcite in natural, passive light, well outside the visible spectrum, and combine active (lidar) and passive remote-sensing technologies to conduct remote mineralogical mapping linked to diagenetic alteration of carbonates. Through the combination of hyperspectral image processing and shape/texture analysis of terrestrial lidar data, a quantitative, multiscale facies map was generated in three-dimensional, geographically rectified space.

Seismic architecture of a Lower Cretaceous platform-to-slope system, Santa Agueda and Poza Rica fields, Mexico

Two three-dimensional seismic data sets over the Albian western Golden Lane margin and time-equivalent basinal deposits of Poza Rica field allowed us to investigate the linked architecture of a steep-sided carbonate platform (El Abra Formation) and a thick accumulation of redeposited carbonate sediment at the toe of the slope and in the basinal area (Tamabra Formation). Regional seismic cross sections show that the most aggrading Albian platform has an eroded platform top, a scalloped margin, and a channelized slope that are equivalent to a 20-km (12.4-mi)-wide, westward-thinning, thick toe-of-slope apron made of chaotic, contorted, mounded, moderate- to high-amplitude reflections. Detailed reflection geometries in the Albian toe-of-slope and basinal deposits consist of chaotic to short, discontinuous, low-amplitude reflection at the toe of the slope of the Golden Lane platform, laterally changing to a discontinuous mounded, shingling reflection, which ultimately turns into high-amplitude parallel reflections. We interpret this lateral change to reflect the seismic signature of the change from the block- and debris-flow–dominated toe-of-slope area, to debris-flow and concentrated density flow deposits in the basin that ultimately grade laterally into pelagic deposits. On a flattened seismic slice, mounded reflections correspond to lobate to fan-shaped seismic events several kilometers wide that are interpreted as a carbonate basin-floor fan. Comparison between core and seismic data shows a dominance of debris flows in the lower two Albian sequences (Albian 1 and Albian 2) that grade vertically into more lobate concentrated density flows and turbidites in the upper two Albian sequences (Albian 3 and Albian 4). Seismic data used in this study, combined with core observations, do not support the interpretation of the Albian Tamabra Formation being of shallow-water origin. Seismic features identified as basin-floor fan, channel, and debris-flow deposits have a shape and size that are similar to those of other redeposited basinal carbonate deposits elsewhere. The seismic architecture shows that the Poza Rica field is a typical example of thick accumulation of grainy porous carbonate deposits in a basinal setting. This example shows the potential of a large hydrocarbon accumulation in a tectonically modified stratigraphic trap around shallow-water carbonate platforms.

Burial diagenesis in the Upper Devonian reef complexes of the Geikie Gorge region, Canning basin, Western Australia

The Devonian carbonates of the Geikie Gorge region, Canning basin, have undergone a long and complex diagenetic history that began in Devonian seawater with extensive marine cementation of platform-margin lithologies. Devonian-Lower Carboniferous burial diagenesis was the most important porosity occluding episode because almost all primary porosity was destroyed by equant calcite cements (nonluminescent to brightly luminescent to dully luminescent) during this interval. Dolomitization and consequent secondary porosity development also occurred during early burial diagenesis. The distribution and geochemistry of the major calcite cements and dolomite types are consistent with these phases having been precipitated from connate marine or basinal brines. Karstification and mi or calcite cementation took place during Late Carboniferous subaerial exposure. Minor calcite cementation occurred during Permian-Cenozoic burial, predominantly in secondary porosity within pervasively dolomitized lithologies. Karstification, dedolomitization, and calcite recrystallization took place in association with Cenozoic meteoric diagenesis. Secondary moldic and intercrystalline porosity within the completely dolomitized lithologies were the longest lived porosity types in the carbonates. Some secondary porosity escaped both Devonian-Carboniferous and Permian-Cenozoic burial cementation, probably due to a lack of nucleation sites for calcite cements within completely dolomitized lithologies.

Play Analysis and Digital Portfolio of Major Oil Reservoirs in the Permian Basin: Application and Transfer of Advanced Geological and Engineering Technologies for Incremental Production Opportunities

The Permian Basin of west Texas and southeast New Mexico has produced >30 Bbbl (4.77 x 10{sup 9} m{sup 3}) of oil through 2000, most of it from 1,339 reservoirs having individual cumulative production >1 MMbbl (1.59 x 10{sup 5} m{sup 3}). These significant-sized reservoirs are the focus of this report. Thirty-two Permian Basin oil plays were defined, and each of the 1,339 significant-sized reservoirs was assigned to a play. The reservoirs were mapped and compiled in a Geographic Information System (GIS) by play. Associated reservoir information within linked data tables includes Railroad Commission of Texas reservoir number and district (Texas only), official field and reservoir name, year reservoir was discovered, depth to top of the reservoir, production in 2000, and cumulative production through 2000. Some tables also list subplays. Play boundaries were drawn for each play; the boundaries include areas where fields in that play occur but are 1 MMbbl (1.59 x 10{sup 5} m{sup 3}) was 301.4 MMbbl (4.79 x 10{sup 7} m{sup 3}) in 2000. Cumulative Permian Basin production through 2000 from these significant-sized reservoirs was 28.9 Bbbl (4.59 x 10{sup 9} m{sup 3}). The top four plays in cumulative production are the Northwest Shelf San Andres Platform Carbonate play (3.97 Bbbl [6.31 x 10{sup 8} m{sup 3}]), the Leonard Restricted Platform Carbonate play (3.30 Bbbl 5.25 x 10{sup 8} m{sup 3}), the Pennsylvanian and Lower Permian Horseshoe Atoll Carbonate play (2.70 Bbbl [4.29 x 10{sup 8} m{sup 3}]), and the San Andres Platform Carbonate play (2.15 Bbbl [3.42 x 10{sup 8} m{sup 3}]).