Figure 4 - uploaded by Matthew Wehner
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f Image of hummocky cross-stratification (HCS) and swaley cross-stratification (SCS), both are types of wave ripples, from Antonio Canyon, Terrell County. A), some SCS is in the center part where the sedimentary structure was destructive by extensive scouring and partial refilling with wave ripples. In the upper part are constructive features (hummocks) representing HCS. B), several stacking patterns of scouring, coarse lags (marked by white arrow) with abundant bivalve shells, and finally parallel lamination. Scale is in cm. C), low-angle laminated wackestone/packstone ripples, sometimes connected and sometimes isolated, interbedded with mudstones. Scale is in cm.
Source publication
The sequence-stratigraphic framework of the Cenomanian-Turonian Eagle Ford Group, defined previously from outcrops and a borehole in Lozier Canyon (Terrell County, west Texas), is correlated to outcrops in the Hot Springs area of Big Bend National Park (Brewster County, Texas) and a core in Webb County (Maverick Basin) in south Texas. This correlat...
Contexts in source publication
Context 1
... drawn to show the orientation of laminations and truncation of hummock cross-bedding bedsets consists of amalgamated silt-to coarsegrained foraminiferal grainstone (and packstone) containing bedforms that consists of an erosional base, low-angle laminae, and laminae that thicken toward the thicker part of ripple and thins toward the ripple trough (Figs. 4A and 4C). The ripples display a convex and/or concave internal laminae structure in each bedset. These concavo-convex symmetric structures occur in a repetitive stacking pattern consisting of: an erosional base, a lag of inoceramid valves, planar lamination, and either HCS/SCS or convoluted laminations. Sometimes the bedforms are connected or ...
Context 2
... structures from units A, B, and E of the Eagle Ford Group (Fig. 4) include characteristics such as: symmetric ripple, both convex and concave internal laminae geometry (concavo-convex symmetric ripples), and thickening and thinning of laminae consistent with the descriptions of hummocky ( Harms et al., 1975) and swaley cross-stratification ( Leckie and Walker, 1982). These structures were initially ...
Similar publications
Upper Cretaceous (Cenomanian-Turonian) Eagle Ford Group equivalent strata in west Texas (Ojinaga, Boquillas Formations) can be correlated with subsurface Eagle Ford Group strata in south Texas using hand-held spectral gamma-ray logs on outcrops, biostratigraphy, stable isotopes and U/Pb geochronology. These correlations indicate that some of these...
Citations
... The deposition of the Eagle Ford sediments represented a long-term flooding (mid-Cenomanian through the end of Turonian) of the Comanche Reef platform throughout most of Texas during the mid-Cretaceous. The basins on the Comanche platform and its margins initially experienced some restriction and episodic anoxia (Wehner et al., 2015). The Eagle Ford sediments primarily consist of alternating mudstone or marlstone with limestone, along with organic carbon content up to 10 wt. ...
... % TOC (Sun et al., 2016), and occasional bentonitic ash beds. The LEF appears to have been deposited during basin restriction, storm deposition, and episodic anoxia; these factors contributed to the ultimate incorporation of organic matter (Wehner et al., 2015). The variations of depositional condition make the Eagle Ford Formation highly heterogeneous in the vertical dimension (at the centimeter and decimeter thickness scales). ...
Integration of geomechanical, geological, geochemical, and petrophysical characterization is critical to enhance production from organic-rich mudrocks. This paper introduces an integrated rock classification method in the Eagle Ford Formation shales and marls in southern Texas, consisting of organic-rich fossiliferous marine shale deposited during the Late Cretaceous. A joint inversion of triple-combo, spectral gamma-ray, and elemental capture spectroscopy logs was initially conducted to estimate volumetric concentrations of minerals, porosity, and fluid saturation. Effective elastic properties such as Young’s modulus and Poisson’s ratio as well as minimum horizontal stress were then estimated as a function of depth. Rock classification was finally performed based on geologic texture and geochemical properties, as well as well-log–based estimates of petrophysical and geomechanical properties.
The introduced method was applied to two wells located in the oil window of the Eagle Ford Formation (average gas–oil ratio of less than 2000 SCF/STB). The results of the integrated rock classification demonstrated similar organic richness and petrophysical properties in both wells. However, the geomechanical rock classification as derived from in situ stress profiles suggests higher proportions of the rock class with better completion quality (55% of net thickness in one well vs. 34% in the other well). A 90-day report on the cumulative oil production of this well shows an additional 11,000 bbl (i.e., 20% more oil production) compared to the second well. This observation was accounted for by geomechanical properties and the distribution of rock classes that differentiate reservoir quality and production.
Upper Cretaceous (Cenomanian-Turonian) Eagle Ford Group equivalent strata in west Texas (Ojinaga, Boquillas Formations) can be correlated with subsurface Eagle Ford Group strata in south Texas using hand-held spectral gamma-ray logs on outcrops, biostratigraphy, stable isotopes and U/Pb geochronology. These correlations indicate that some of these units are incompletely recorded even though they do not preserve evidence for long-term exposure.
The Ojinaga Formation outcrops in west Texas records only Cenomanian strata based on its high uranium abundance throughout and biostratigraphy. Multiple ash beds correspond to thorium spikes. Sedimentary structures are limited to hummocky cross stratification (HCS) within the lower 30 feet. In Big Bend National Park two detailed measured sections (Hot Springs and Ernst Tinaja) of Boquillas Formation strata also suggest much of the Cenomanian is recorded but the Turonian portion is only partially preserved; specifically, neither of these sections preserves strata equivalent to Ocean Anoxic Event 2 (OAE2) that spans the Cenomanian-Turonian boundary. The Lower Eagle Ford Group of each section has elevated uranium abundance and contains abundant HCS. The Upper Eagle Ford Group at both sections has little uranium and few wave generated structures.
At Lozier Canyon in Terrell County (west Texas) approximately 80 miles (130 km) to the east of Big Bend National Park, an almost complete section of the Eagle Ford Group is well preserved. Here the Lower Eagle Ford Group has abundant uranium and molybdenum (Mo), whereas the Upper Eagle Ford Group has much less of these two elements. The Lower Eagle Ford Group here contains abundant HCS and transitions upward into bioturbated skeletal wackestone/packstone. The stable isotope excursion associated with OAE2 is truncated at this section indicating a short-lived unconformity in this section.
Subsurface correlation of multiple cores and wireline logs of the Eagle Ford Group in south and west Texas indicate that its thickness increases to the south into the Maverick Basin. The Lower Eagle Ford Group commonly has abundant uranium and locally preserved abundant thorium spikes. HCS is common in many of the cores of the Eagle Ford Group and bioturbation is rare. Biostratigraphy and stable isotope analysis indicates that many of these cores, even though they are thicker do not preserve the entire OAE2.
Surface to subsurface correlations indicate that the Eagle Ford Group was deposited across a broad area of Texas that deepened to the south and west. Abundant HCS and increased bioturbation updip on the shelf indicate much deposition there occurred within storm wave base. Geochemical proxies (U, Mo) indicate much of the deposition occurred within a zone of shallow-water euxinia. Lastly, subtle surfaces record prolonged periods of non- deposition, the processes forming these surfaces are heretofore unknown as there is little or no physical evidence for subaerial exposure.
The Late Cretaceous Eagle Ford Group is a prolific unconventional oil and gas source rock play. Southwest of the San Marcos Arch Eagle Ford strata are carbonate-rich consisting predominantly of carbonate-rich (up to 40 wt % calcium) organic-rich mudstone beds interbedded with foraminiferal packstone and grainstone beds. The packstone and grainstone beds commonly contain hummocky and wavy cross-stratification (HCS and WCS) suggesting deposition occurred within storm wave base (10’s of m’s water depth). Rare Chondrites on the top of HCS and WCS beds indicates the storms allowed oxygen to episodically reach the sea floor. However, geochemical tracers (Mo, V) and absence of bioturbation in carbonate-rich mudstone indicate that deposition of these facies occurred in anoxic and/or euxinic conditions. A hand-held gamma-ray scintillometer that measures the total gamma rays and their component parts (U, Th, K) provides unique signatures that allow correlations with abundant subsurface logs. Numerous widespread ash beds within the Eagle Ford Group also are providing a geochronologic framework to correlate this unit from the surface to the subsurface. The positive carbon isotope fluctuation associated with Ocean Anoxia Event 2 (OAE2), near the Cenomanian-Turonian boundary also provides another avenue for correlation from the outcrops to the subsurface. However, a regionally extensive unconformity near the Cenomanian-Turonian boundary suggests caution in using this as a stand-alone proxy.
The Eagle Ford Shale is an organic-rich marine carbonate-dominated mudrock located in the Gulf Coast Region, Texas. This mudrock was deposited during the Late Cretaceous and is composed of two main members: the lower and the upper Eagle Ford, which are associated to marine transgression and regression episodes respectively. Petrographic analysis of two cores from wells located in Gonzales (Well #1) and La Salle (Well #2) counties indicates that the Eagle Ford Shale has a strong cyclic character. Eight different facies and six microfacies were recognized at different well depths. A paleontologic study was carried out to better characterize the Eagle Ford Shale, where both benthonic and planktonic microfossils were identified. The most abundant microorganisms are foraminifera, although echinoderms, bivalves, fish bones, radiolaria, coccoliths and calcispheres are also present. This diversity in microfossils reinforces the cyclic character of the Eagle Ford. Contrary to the commonly believed idea that marine transgression implies deep marine anoxic conditions, this study indicates that it is not necessarily true. Instead, marine transgression may also lead to shallow marine conditions where benthonic organisms are abundant, similar to what is observed in the lower Eagle Ford at Well #2. Pore system was characterized at both well locations by combining water immersion porosimetry (WIP), mercury injection capillary pressure (MICP) and scanning electron microscopy (SEM). Total porosity results indicated that WIP overstates the total porosity when clay content is above 50 wt. %. MICP shows more reliable total porosity values where mineralogy does not apparently affect the experiment final results. Total porosity ranges between 0.32-10.27 percent, where most pore throats diameters fall within the nanopore to micropore size range. Intraparticle pores within the organic matter are dominant in the Eagle Ford, especially in Well #2 where thermal maturity is higher compared to Well #1. Hydrocarbon potential was studied by using both the Basic/Bulk-Rock and Institut Français du Pétrole Energies Nouvelles (IFPEN) Shale Play Rock-Eval pyrolysis methods. The IFPEN Shale Play method allows to a better quantification of the hydrocarbons in unconventional plays compared to the Basic/Bulk-Rock method. The average increase in hydrocarbons quantification using the new method is 25.69 percent. The intervals with higher hydrocarbons content are the lower Eagle Ford at Well #2 location (6.68-22.69 mg HC/g rock) and the lower portion of the upper Eagle Ford at Well #1 location (6.74-8.22 mg HC/g rock). Pyrolysis analysis was coupled with petrographic study on SEM. Detrital and secondary organic matters were recognized in both well locations, where secondary organic matter is dominant. A strong correlation between hydrocarbon potential and foraminifera test chamber cementation was found. Well intervals where foraminifera test chambers are filled with kaolinite and secondary organic matter present higher oil-in-place (OIP) compared to those intervals where the test chambers are cemented with calcite. Likewise, intraparticle organic matter porosity is denser at those intervals with higher potential in terms of OIP.
