Browsing by Subject "Facies variability"
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Item High resolution stratigraphy and facies architecture of the Upper Cretaceous (Cenomanian-Turonian) Eagle Ford group, Central Texas(2012-08) Fairbanks, Michael Douglas; Fisher, W. L. (William Lawrence), 1932-; Ruppel, Stephen C.Heightened industry focus on the Upper Cretaceous (Cenomanian-Turonian) Eagle Ford has resulted from recent discoveries of producible unconventional petroleum resource in this emerging play. However, little has been published on the facies and facies variabilities within this mixed carbonate-clastic mudrock system. This rock-based study is fundamental to understanding the controls, types, and scales of inherent facies variabilities, which have implications for enhanced comprehension of the Eagle Ford and other mixed carbonate-clastic mudrock systems worldwide. This study utilizes 8 cores and 2 outcrops with a total interval equaling 480 feet and is enhanced by synthesis of thin section, XRD, XRF, isotope, rock eval/TOC, and wireline log data. Central Texas Eagle Ford facies include 1) massive argillaceous mudrock, 2) massive argillaceous foraminiferal mudrock, 3) laminated argillaceous foraminiferal mudrock, 4) laminated foraminiferal wackestone, 5) cross-laminated foraminiferal packstone/grainstone, 6) massive bentonitic claystone, and 7) nodular foraminiferal packstone/grainstone. High degrees of facies variability are observed even at small scales (50 ft) within the Eagle Ford system and are characterized by pinching and swelling of units, lateral facies changes, truncations, and locally restricted units. Facies variability is attributed to erosional scouring, productivity blooms, bottom current reworking, and bioturbation. At the 10-mile well spacing scale and greater, the data significantly overestimates intra-formational facies continuity but is successful in defining the following four-fold stratigraphy: The basal Pepper Shale is an argillaceous, moderate TOC, high CGR and GR mudrock. The Waller Member is a newly designated name used in this study for an argillaceous and foraminiferal, high TOC, massive mudrock with a generally moderate CGR and GR profile. The Bouldin Member is a high energy, carbonate-rich (foraminiferal), low TOC, low and variable CGR but high GR zone. Finally, the South Bosque Formation is an argillaceous and foraminiferal, moderate TOC, massive and laminated mudrock with a moderate CGR and GR signature. GR logs alone are inadequate for determination of facies, TOC content, depositional environment, and sequence stratigraphic implications. Using integrated lithologic, isotopic, and wireline log data, cored wells in the study area are correlated across the San Marcos Arch. Geochemical proxies (enrichment in Mo, Mn, U, and V/Cr) indicate that maximum basin restriction occurred during deposition of the Bouldin Member. Bottom current activity influenced depositional processes and carbonate sediment input was driven by water column productivity. These primary controls on Eagle Ford stratigraphy and character are independent from eustatic fluctuation, rendering classical sequence stratigraphy unreliable.Item Shelf-to-basin architecture and facies variability of a Cretaceous intrashelf basin in the northwest Gulf of Mexico(2015-12) Sitgreaves, Jeffrey Robert; Kerans, C. (Charles), 1954-; Loucks, Robert G; Fisher, William LThe geomorphic expression of intrashelf basin systems (ISBs) and their associated facies patterns is extremely subtle, with shelf-to-basin dip angles that can average 0.3° across the slope profile. This presents an issue to stratigraphers working to understand facies variability at the reservoir-scale because the changes in stratal geometries at the shelf-to-basin transition will occur beneath the resolution of conventional subsurface datasets. Exposures along the Pecos River Canyon provide a unique opportunity to observe the transition from grain-dominated facies of the ramp crest into planktonic foraminifera mudstones/wackestones of the intrashelf basin. For this study, 475 m of detailed sections were collected at five localities and integrated with a high-resolution 3D digital outcrop model (DOM) to document the relationship between vertical facies successions and stratal geometries of the intrashelf basin profile. The high-resolution DOM provides the ability to accurately interpret the subtle depositional dips of the shelf-to-ISB profile that range from less than 0.1° to 0.7°. The development of the differential topography and facies changes associated with the formation of the Maverick ISB is attributed to differential sediment accumulation rates between active rudist-skeletal shoal formation versus deeper-water foraminiferal mudstones of the basin-center. Rudist bank deposition early in the Albian 6 Composite Sequence formed the positive topographic relief (1-3m) that led to the localization of rapid shallow-water sediment accumulation. After the development of subtle topographic expression, ensuing changes in relative sea-level promoted the development of ISB margins that were dominated by rudist faunal assemblages. The development of the ISB margin increasingly led to the differentiation between the grain-dominated facies along the margin and deposition of globigerinid mudstones in the basin-center. The extensive and largely undeformed exposures along the lower Pecos River Canyon and adjacent Amistad Reservoir provide clear evidence of the constructional differential-accumulation-driven formation for the Maverick ISB. Similar constructional models are likely for the East Texas and Fort Stockton ISBs on the Texas Comanche Shelf. Similar constructional progressions have been called on for the Bab intrashelf basin and the Natih-E Formation in the Cretaceous of the Middle East.