ALBERT

Description: :  Many studies document the occurrence of carbonate progradational successions that are driven by overall accommodation-limited conditions and represent periods when substantial volumes of sediment are stored in slope settings. However, few have reported on the detailed internal anatomy and components of prograding clinothems, and furthermore, little is known about how environmental parameters affect the development of carbonate margins and slopes when superimposed on the long-term, low-accommodation signal. Upper Devonian (Frasnian and Famennian) carbonate outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, offer the examination of reefal margin and foreslope development in a variety of settings, one of which is long-term (〉 10 Ma) progradation during the late Frasnian and Famennian. In the early phases of progradation, a global biotic crisis and faunal overturn occurred around the Frasnian–Famennian (F-F) boundary, and indications of global climatic cooling are interpreted at points throughout the Late Devonian. These extrinsic and intrinsic controls provide ecologic context and a stratigraphic framework to assess carbonate margin and foreslope architectures and facies distributions in order to better characterize and predict their complex heterogeneity. The outcrop dataset collected includes measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs from the South Lawford Range, Windjana Gorge, and Dingo Gap areas. The outcrop observations highlight distinctive changes in margin style, foreslope composition, stratigraphic packaging, and strike variability within the contexts of the F-F biotic crisis, long-term highstand (HST) progradation, and Late Devonian climatic changes. Intervals before and after the F-F boundary exhibit notable increases in downslope calcimicrobial boundstone encrustation and evidence for waning foreslope contributions from other, previously flourishing, carbonate factories, reflecting stressed ecological conditions and consequent calcimicrobial opportunism during gradual buildup and recovery periods that bracket the extinction. The Famennian slope (post-effects related to the F-F crisis) can be subdivided into a calcimicrobial boundstone-dominated upper slope with a highly complex downdip transition into a mixed debris- and grain-dominated middle slope, and a silt-dominated lower slope to toe of slope, and exhibits a highly complex lateral heterogeneity associated with multiple contributing sediment factories and temporal partitioning of resedimentation. A repeated stacking pattern is observable in the Famennian middle-slope setting, consisting of upward successions of debris followed by a gradation from silt-dominated deposits into grain-dominated deposits; we here interpret these as the expression of high-frequency sequences superimposed upon the longer-term progradation, and a function of transitional (greenhouse to icehouse) climates. These findings offer predictive relationships that link the internal depositional heterogeneity of prograding slope systems to the interplay of long-term accommodation drivers, ecological conditions affecting carbonate sediment factories, and climatic conditions that control the development of high-frequency sequences.

Description: 〈span〉〈div〉ABSTRACT〈/div〉The three-dimensionally complex, highly progradational mixed siliciclastic–carbonate strata of the San Andres and Grayburg Formations have long been the backbone of conventional hydrocarbon reservoir production from the Permian Basin, and significant recovery continues via waterflooding and CO〈sub〉2〈/sub〉 injection. Besides, nonreservoir equivalents of these formations have recently taken increasing significance as produced water disposal targets. However, seismic-stratigraphic interpretations are challenged by complex internal shelfal-stratal geometries and numerous laterally continuous but vertically thin fluid barriers in overlying platforms. We built a three-dimensional (3-D) geocellular model of Guadalupian 8–13 high-frequency sequences (G8–G13 HFSs) and then conducted forward seismic modeling (35-Hz 0° phase). This allows investigations on the validity of applying conventional reflection-geometry–based interpretation to delineate the G9 HFS top and base, which can potentially serve as bounding/constraining surfaces for upper San Andres shelf–Grayburg platform reservoirs. This study contributes to 3-D modeling methodologies by introducing a query tree to select geostatistical methods for modeling dual-scale heterogeneities and by integrating data from diverse sources for seamless and realistic 3-D models. Our seismic-stratigraphic evaluation demonstrates that conventional reflection–geometry-based interpretation does not adequately resolve the G9 top and base; deviations from the geocellular model reach up to 80 m (260 ft) and are thus well beyond the maximum acceptable error limits of ±0.5 wavelength. We suggest improving conventional interpretations of the G9 base by selective interpolation or mixed-polarity event picking near the error-prone shelf margin and upper slope. Besides, instead of picking the highly discontinuous seismic peak as G9 top, bulk-shifting of a shallower trough horizon near actual G10 top should deliver a more accurate surface representing G9 top.〈/span〉

Description: : Syndepositional fractures are common in Upper Permian strata of Dark Canyon, Guadalupe Mountains, New Mexico, USA, and are typically oriented parallel to the platform-margin trend with subvertical traces. Fracture apertures range from millimeter- to meter-scale with sediment, microbialite, breccia, and cement infill. Syndepositional normal faults are less frequent, and commonly are filled with fault-breccia and flanked by damage zones. Syndepositional fault offsets range from tens of centimeters up to 18 meters. In some instances, fault offsets were large enough to alter sedimentation patterns and stratal architecture of the Tansill shelf in the form of growth monoclines or small-scale grabens. Evidence for flow of dolomitizing fluids along syndepositional deformation features consists of narrow (centimeter- to meter-scale) dolomite halos along the fractures and faults, and dolomite bodies that extend laterally for several meters to tens of meters away from faults and fractures. Halos are interpreted to have formed where deformation features crosscut low-permeability strata. Dolomite bodies formed where deformation features intersected more permeable strata and dolomitizing fluids flowed laterally into the surrounding host rock. Isotopic data from individual fracture-controlled dolomite bodies indicate temporal variability in the dolomitizing fluids, suggesting that those fluids moved through different syndepositional fractures systems at different times.

Description: :  Carbonate reefal margin and foreslope settings are characteristically heterogeneous and difficult to predict due to a spectrum of sediment source factories, resedimentation processes, resultant deposit types, and controlling parameters. In particular, the effects of changes in long-term accommodation on the composition, architecture, and sediment distribution patterns of carbonate margin–foreslope–basin systems are poorly understood. Upper Devonian (Frasnian) outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, were investigated to assess the development of reefal margin and foreslope settings during long-term 1) platform backstepping with aggradational pulses, and 2) across the transition from platform aggradation to progradation. Measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs were collected from the South Lawford Range and Windjana Gorge areas. The exposures reveal distinctive differences in foreslope grain composition, deposit characteristics and proportions, margin morphology, and stratigraphic expression 1) during platform evolution between backstepping events, and 2) depending on position within the long-term accommodation setting. Between backstepping events, aggradational margins can be classified as "growth escarpments" with associated grain-dominated, onlapping foreslope deposits. Margins across the long-term transition from aggradation to progradation evolved from erosional escarpments with onlapping debris deposits to accretionary, interfingering configurations. Development of growth escarpments between backstepping events was a function of vertical reefal growth from sustained high accommodation conditions during the long-term transgressive systems tract, coupled with a Frasnian reefal assemblage that responded to light and tracked relative sea level. This net vertical reefal growth also resulted in relative margin stability and the deposition of grain-dominated foreslopes. Conversely, margins were highly unstable and underwent repeated failure across the long-term aggradation-to-progradation transition, reflecting a lack of underlying substrate to support basinward advance, and resulting in debris-dominated foreslopes. These observations provide relationships that predict margin and foreslope associations of facies-scale heterogeneity and seismic-scale geometry within a low-frequency sequence stratigraphic framework.

Description: 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.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Fossilized reefs can preserve critical information about changes in marine environments over a relatively short period of time. The interpretation of these changes is often hindered by the complexity of reef growth with respect to architecture, biotic zonation, and time. High-resolution mapping and data collection incorporating both sequence stratigraphical and paleoecological principles are needed to document the architectural complexity of reef development. To demonstrate this, we present a case study in which both principles are integrated to build a new stratigraphic framework for an Albian-aged rudist-coral patch reef outcrop (Paul Spur, Bisbee, AZ, USA). The dataset reveals that the outcrop preserves five stages of development: (1) initial shoal deposition; (2) pioneer reef growth; (3) reef diversification; (4) reef hiatus; and (5) rudist shoal development. These stages represent periods of deposition and reef growth within high-frequency transgressive-regressive sequences. Interpretations of sedimentological and paleoecological data are then used to demonstrate the variable influence of different environmental controls on reef growth. Prevailing wind and current direction act as higher order controls on overall reef architecture by influencing windward-leeward asymmetry. Fluctuations in relative water depth as well as sedimentation rate, source, and type is an important influence on reef community and growth habit. Though corals and rudists cohabited during much of the reef's history, corals dominated when water depth was greater and external sediment influx lesser, whereas rudists dominated in shallow water depths and during periods of high external sediment influx. This work demonstrates that detailed evaluation of stratigraphy and paleoecology, as well as careful consideration of timelines and heterogeneity, is essential for building an accurate stratigraphic framework that allows a more thorough understanding of processes driving reef growth.〈/span〉

Description: Micropore-dominated carbonate reservoirs remain challenging for accurate hydrocarbon evaluation and production because conventional reservoir models using depositional textures and petrophysical properties to distribute porosity and permeability cannot be applied. Nevertheless, understanding the distribution of pore systems and predicting the fluid flow behavior of microporous reservoirs is fundamental because micropores constitute a significant percentage of the total porosity and storage capacity. We present the results from an integrated study on the producing micropore-dominated Word field characterized by a facies-independent, diagenetically controlled pore system that approaches 100% microporosity. Four cored wells through the Albian Edwards Formation were described and correlated using stacking patterns and vertical facies trends; pore type characterization was done through thin section petrography, routine core analyses, scanning electron microscopy, and mercury injection capillary pressure data. This study is an example of a permeable reservoir in which intergrain pores are cemented during burial diagenesis and micropores, being more resistant to cementation, remain open to depths greater than 4000 m (13,000 ft). A unique relationship exists between porosity, permeability, median pore throat size, and microcrystalline textures, independent of facies and fabrics. Cumulative gas production data show there is a correlation between the total porosity and the structural position of the wells: wells high on the structure have the highest production. We demonstrate that an equally well–connected micropore network exists in mud-dominated rocks via the matrix and via grain-to-grain contacts in grain-dominated rocks. The here described intragrain micropore network through grain-to-grain contacts in cemented grainstones is a new carbonate flow path that will likely become more important as more unconventional carbonate reservoirs are explored.

Description: The Lower Cretaceous stratigraphic section in the northern Gulf of Mexico (GOM) comprises several well-documented carbonate systems that have prominent shelf-margin buildups, including the Berriasian Knowles Limestone ramp and shelf system, Barremian Sligo Formation shelf system, Aptian Pearsall Formation ramp system, and shelf systems of the Albian Glen Rose, Edwards, and Stuart City Formations. Two lesser-known but large-scale Lower Cretaceous Valanginian carbonate shelf to shelf-margin systems are documented in this investigation using core, wireline-log, and regional two-dimensional seismic data. The platform-margin succession of the older Calvin section comprises a shelf-margin buildup (back reef, reef, and fore reef facies) that may be as thick as 2000 ft (610 m). The seaward margin of the shelf, as displayed by seismic data, suggests shelf-to-basin relief of as much as 1000 ft (305 m) and margin-to-lagoon relief of 200 to 500 ft (60 to 140 m) (relief estimates are not decompacted). Major reef-building organisms are Lithocodium , stromatoporoids, and corals. The Winn limestone is younger than the Calvin limestone, and at the shelf margin, it may have been 600 to 800 ft (180 to 245 m) thick. It is composed of organisms similar to those of the Calvin reef complex. Considering the vertical position of the Calvin platform, succeeding the major bypass of Calvin lowstand siliciclastic sands, the platform probably records a transgression, which is consistent with its highly aggradational depositional style. The Winn shelf margin was initiated landward of the Calvin shelf margin and did not prograde as far seaward as the Calvin shelf margin. The recognition of these two Lower Cretaceous carbonate systems is important because they further detail the stratigraphic history and architecture of this deeply buried, lowermost Lower Cretaceous section in the GOM. These formations are also potential deep-gas, tight-carbonate (low-porosity) plays.