William C. Dawson

Shale Microfacies: Eagle Ford Group (Cenomanian-Turonian) North-Central Texas Outcrops and Subsurface Equivalents

Abstract The Eagle Ford Group is a mixed siliciclastic/carbonate unit that records a Late Cretaceous (Cenomanian to Turonian) transgression. A major condensed interval (cycle 2.5 of the UZA-2 supercycle) occurs within the Eagle Ford Shale. Eagle Ford strata are overlain disconformably by the Austin Chalk. The Austin/Eagle Ford contact represents the Turonian/Coniacian boundary (89 MA). The Cenomanian/Turonian (92 MA) boundary occurs within the Eagle Ford Group. Regionally, Eagle Ford strata consist of two major depositional units: a lower (transgressive) unit dominated by dark well-laminated shales (exhibiting only minor evidence of bioturbation); and an upper (regressive) unit consisting of thinly interstratified (high-frequency cycles) shales, limestones, and carbonaceous quartzose siltstones. Petrographic characterization of shales from Eagle Ford outcrops near Austin, Waco and Dallas, Texas and subsurface equivalents in a core (Getty #1 J. T. Wilson) from LaSalle County, Texas reveals six microfacies: 1) pyritic shales; 2) phosphatic shales; 3) bentonitic shales; 4) fossiliferous shales; 5) silty (quartzose) shales; and 6) bituminous claystone and shales. Transgressive Eagle Ford shales consist mainly of microfacies 1, 4, and 6. The Eagle Ford condensed interval consists of microfacies 1, 2 and 3, and highstand Eagle Ford shales are comprised of microfacies 4 and 5. Eagle Ford microfacies exhibit distinctive sedimentological aspects and have source rock characteristics that vary systematically within a sequence stratigraphic framework. Transgressive Eagle Ford shales (occurring below the condensed interval) have optimum oil-prone source potential, whereas overlying regressive shales are gas-prone. Transgressive and condensed Eagle Ford strata represent poorly oxygenated low-energy, (below storm wave base) marine paleoenvironments of deposition. In contrast, overlying regressive Eagle Ford lithofacies accumulated in higher energy (above storm wave base), well-oxygenated, shallow marine paleoenvironments.

Textural and sequence-stratigraphic controls on sealing capacity of Lower and Upper Cretaceous shales, Denver basin, Colorado

Shale units can be important barriers to fluid flow in sedimentary basins and commonly serve as seals to petroleum reservoirs. Little is known, however, about the controls on shale permeability. Consequently, variation in seal competency is one of the greatest risk factors associated with petroleum exploration.Here, we examine possible controls on sealing capacity in two Cretaceous marine shale units in the Denver basin, Colorado. Sealing capacity, as determined by mercury injection–capillary pressure analysis, is compared to several textural and compositional parameters and to sequence-stratigraphic setting. These two shale units display highly variable sealing capacity, even between some adjacent samples. This suggests that variability in some small-scale shale characteristics may strongly influence sealing capacity. The best seals are generally in transgressive systems tracts, especially within or immediately below condensed sections.Textural characteristics of shale appear to be especially important in determining sealing capacity. In particular, well-sorted pore-throat sizes and well-developed bedding-parallel preferred orientation of flattened organic matter particles strongly favor high sealing capacity. High degrees of bioturbation degrade sealing capacity, possibly by disrupting preferred orientation and by increasing variability in grain size and hence in pore-throat sorting. Preferred orientation of matrix clays parallel to bedding also appears to increase with increasing sealing capacity, but is probably less important than the preferred orientation of organic matter.Compositional characteristics are generally less important than textural characteristics in determining sealing capacity in these shale units. Neither silt content nor cement content appears to be important to sealing capacity in these shale units. Total organic carbon is generally high in samples with good sealing capacity, but can be either high or low where sealing capacity is poor.Overall, the variables that most strongly favor high sealing capacity, pore-throat sorting, organic matter bedding-parallel preferred orientation, and low bioturbation, are most likely in anoxic, deep-water settings, hence, the association between good seals and condensed sections.

Diagenetic and Sedimentologic Aspects of Eagle Mills-Werner Conglomerate Sandstones (Triassic-Jurassic), Northeast Texas

ABSTRACT The Eagle Mills-Werner sequence occurs in deep wells (12,000 to 18,000 ft.) in northeast Texas and southwest Arkansas. Eagle Mills sandstones unconformably overlie the Paleozoic complex and disconformably underlie the Werner Conglomerate. Werner-Louann evaporites overlie Eagle Mills-Werner siliciclastics. Basaltic sills and dikes transect these sandstones and conglomerates. Eagle Mills-Werner strata consist of greenish-gray conglomerates and green, red, and pink, coarse-grained, lithic-feldspathic arenites which are interbedded with red, gray, and green shales and siltstones. Lithic arenites and conglomerates contain volcanic and sedimentary rock fragments. These strata represent nonmarine paleoenvironments developed within grabens and are the earliest record of Gulf of Mexico rifting. Eagle Mills-Werner sandstones have undergone a complex diagenetic history, including: chlorite cementation, compaction, quartz and feldspar overgrowths, early carbonate cementation, dissolution of framework grains, chloritization and albitization of feldspars, late cementation by ferroan calcite and ferroan dolomite and late replacement by anhydrite and pyrite. Pyrobitumens coat early chlorite cements indicating that most diagenesis post-dated hydrocarbon migration. This suite of authigenic phases records progressive burial of Eagle Mills-Werner strata into a high-temperature diagenetic regime where thermochemical sulfate reduction was the dominant process.

Digenesis of Deeply Buried Eagle Mills Sandstones: Implications for Paleo-Fluid Migration and Porosity Development

ABSTRACT Eagle Mills strata (Triassic-Jurassic) unconformably overlie the Paleozoic basement complex and thus, form the basal sedimentary unit within the Gulf of Mexico Basin. Jurassic-aged basaltic dikes and sills have intruded Eagle Mills strata. Werner-Louann evaporates disconformably overlie the Eagle Mills Formation. These sedimentary and igneous rocks preserve the earliest record of Gulf of Mexico rifting. Deeply buried (15,000 to 18,000 ft/4,573 to 5,488 m) Eagle Mills sandstones have subarkosic and sublithic modal compositions. These sandstones exhibit evidence of a complex and prolonged diagenetic history, including: early chlorite cementation; mechanical compaction; quartz and feldspar overgrowths; nonferroan calcite and dolomite cementation; dissolution of framework grains; kaolinite precipitation; ferroan calcite cementation; albitization; saddle dolomite and anhydrite cementation; and pyritization. Pyrobitumens coat early cements indicating that most diagenesis post-dated hydrocarbon migration. Marked shifts in paleo-water chemistry are recorded by diagenetic phases in Eagle Mills sandstones. The Eagle Mills paragenetic sequence is indicative of progressive burial into a high-temperature (>150° C) regime where thermochemical sulfate reduction was the dominant diagenetic process. Pervasive late burial dissolution of detrital feldspars in some Eagle Mills sandstones provides direct evidence for deep-seated sourcing of metal cations which promoted diagenesis of overlying Mesozoic strata. These petrographic data support interpretations of the Mesozoic Gulf Coast Basin which imply vertical mass-transfer of diagenetic fluids along faults.

ABSTRACT: Early Marine Cementation at Parasequence and Sequence Boundaries: Implications for Seal Development and Reservoir Compartmentalization

Tertiary-aged siliciclastic strata in Indonesia (shallow marine) and offshore Angola (deep marine) include numerous carbonate-cemented (calcite and siderite) horizons. These thin (10 to 30 cm) intervals consist of pervasively cemented, fossiliferous, quartzose sandstones and siltstones, which can occur as discrete beds or nodular horizons. Petrographic features (cementation pre-dated significant compaction) and stable isotope geochemistry are suggestive of an early submarine paleoenvironment of precipitation. Well log-scale sequence stratigraphy indicates that of these dense carbonate-cemented horizons are represented by resistivity ‘spikes’ at the bases of channelized (tidal and estuarine) sandstone units. Some maximum flooding surfaces are represented by calcitecemented foraminiferal siltstones. Other carbonate-cemented units containing Glossifungites mark bases of major (third-order) transgressive packages (e.g., 21 ma sequence boundary in Central Sumatra Basin). Such cemented horizons provide a useful datum for field-wide correlation and reservoir-scale mapping.

Austin Chalk (uppermost Santonian) Discontinuity Surface, North Central Texas: ABSTRACT

ABSTRACT The Austin Chalk-Taylor (Ozan) Marl contact marks the Santonian-Campanian boundary which is a regional unconformity in north-central Texas. This distinctive surface has been examined at three localities where it records evidence of complex sedimentologic and diagenetic histories. This surface is highly irregular and has been stained pervasively with limonite. The most conspicuous aspect of the uppermost Austin discontinuity surface is the abundance of phosphatized and pyritized nodules and bioclasts (gastropods, bivalves, corals, coprolites and Baculites sp.). Fish teeth and bone fragments are also present. These nodules and bioclasts have been penetrated by small-diameter chalk-filled borings. Close examination reveals the presence of abundant trace fossils, especially Rhizocorallium jenenese. These trace fossils have been infilled with reddish-brown clay piped downward from the overlying Ozan Marl. Phosphatic nodules and glauconite also occur in these clay-filled burrows. Well-preserved chalk-filled Rhizocorallium jenenese are present locally. The occurrence of Rhizocorallium sp. in the uppermost Austin Chalk is noteworthy because Rhizocorallium has not been recorded in other Cretaceous chalks of North America or Europe. The exquisite preservation of Rhizocorallium in the uppermost Austin Chalk is indicative of a firm ground paleosubstrate. Based on sedimentologic, mineralogic, and paleontologic data, the Austin Chalk-Taylor Marl contact is interpreted as a condensed horizon (omission surface). This phosphatized condensed horizon overlies the Rhizocorallium firm ground. This thin stratigraphic interval records a complex history of soft-sediment bioturbation, early marine lithification, submarine erosion, and mineralization. The amount of erosion along the Austin-Taylor contact in the study area appears to have been relatively minor. Elsewhere in north-central Texas the Austin-Taylor boundary has been subjected to significant erosion. The Austin-Taylor discontinuity surface formed in a relatively shallow marine (inner to mid-shelf) paleo-environment.

Biogenic Mounds and Associated Trace Fossils: Wolfe City Formation (Upper Cretaceous), North-Central Texas

ABSTRACT Small, conical sandstone mounds occur in some outcrops of the lower Wolfe City Formation (Upper Cretaceous) in Ellis and Navarro Counties, North-Central Texas. Biogenic mounds are common in modern shallow-marine environments (e.g., Callianassa mounds) but are rare in the geologic record. These biogenic mounds resemble the ichnogenus Chomatichnus sp. described originally from Carboniferous limestones of Lancashire, England (Donaldson and Simpson, 1962). This ichnogenus occurs as well-defined conical mounds having a central vertical burrow. Wolfe City Chomatichnus consists of calcite-cemented, glauconitic, bioclastic, argillaceous, fine-grained quartzarenite. The central vertical burrow is lined with shell debris; striae radiate from the central burrow. Thalassinoides, Gyrolithes, and Pseudobilobites occur in lower Wolfe City strata with Chomatichnus. The upper Wolfe City contains a profuse ichnoassemblage of Thalassinoides; Pseudobilobites; Gyrolithes; U- and star-shaped trace fossils; large-diameter, sinuous, horizontal burrows; and tripartite digitate burrows. The Wolfe City ichnoassemblage records the activities of infaunal crustaceans in a low-energy shallow-marine paleoenvironment. Chomatichnus has probably been preserved here because of early marine cementation and/or rapid sedimentation (storm) events.

Hardground Petrography and Carbonate Microfacies: Paola Limestone (Upper Pennsylvanian), Southeastern Kansas: ABSTRACT

The Paola Limestone (Missourian) of the Mid-Continent region is the basal carbonate member of the Iola Formation (Kansas City Group). The Paola is a thin (1 to 3 ft; .3 to .9 m) massive layer of bioturbated, fossiliferous (algae, crinoids, and foraminifers) calcilutite containing abundant phosphatic nodules. This distinctive limestone is, according to previous investigators, correlative from Nebraska, southward, into northeastern Oklahoma. The Paola Limestone is overlain (in ascending order) End_Page 447------------------------------ by the Muncie Creek Shale and Raytown Limestone. In a NE-SW outcrop trend across Allen County, Kansas, the Paola Limestone forms the initial substrate on which a phylloid algal buildup developed within the Raytown Limestone. The Paola consists of three distinctive carbonate microfacies (described below). Microfacies 2 overlies microfacies 1; this microfacies association occurs only beneath the phylloid algal buildup. Both exhibit petrographic features indicative of submarine lithification. Northeastward, away from the phylloid algal buildup, microfacies 1 and 2 change abruptly into microfacies 3. Microfacies 1 is a moderately bioturbated pyritized calcilutite with Archaeolithophyllum crusts, Hikorocodium, Tetrataxis, Tuberitina, and low-spired gastropods. This microfacies has a highly irregular (scoured) upper surface that is encrusted by Nubecularia, Archaeolithophyllum lamellosum, and bryozoans and locally penetrated by borings. Microfacies 2 consists of profusely bioturbated, matrix-supported, crinoidal-fusulinid biocalcarenite. Large, bean-shaped, algaloid concretions of Nubecularia and Archaeolithophyllum lamellosum are common accessory components. The large, ramose burrow networks are infilled with microcrystalline dolomite and scattered phosphate nodules; small rugose corals also occur in the burrow fills. Microfacies 3 is a crinoidal-pelletoidal biocalcarenite containing Archaeolithophyllum crusts. Composita, oncolites, productid brachiopods, small gastropods, fenestrate bryozoans, brachiopod and echinoid spines, Nubecularia-encrusted bioclasts, ostracods, and neomorphosed pelecypods shells are accessory components. Baroque dolomite occurs as a filling within phylloid algal blades. Bioturbation textures are present, but sparse, relative to microfacies 1 and 2. Prior to lithification, the hardground (microfacies 1) was bioturbated; following lithification it was scoured, encrusted, and bored. The lithification of microfacies 1 is inferred to have occurred in a submarine environment because: (1) it contains a fauna of encrusting marine organisms and (2) petrographic features indicative of subaerial exposure are lacking. Microfacies 2 is interpreted as a firm ground. Microfacies 3 represents a normal, shallow marine subtidal environment. The recognition of ancient hardgrounds allows a more thorough understanding of the sedimentologic, paleoecologic, and diagenetic histories of carbonate sequences. Submarine diastems also have potential as chronostratigraphic markers. Because petroleum accumulations are commonly associated with diastems, an awareness of these features could provide insights for the location of some obscure hydrocarbon traps. Additionally, hardgrounds can create intraformational permeability barriers; the recognition of such reservoir heterogeneities is essential for optimum hydrocarbon recovery. Detailed petrographic analysis is a prerequisite to the location and understanding of ancient hardground sequences. End_of_Article - Last_Page 448------------