William A. Ambrose

Sequence-stratigraphic controls on complex reservoir architecture of highstand fluvial-dominated deltaic and lowstand valley-fill deposits in the Upper Cretaceous (Cenomanian) Woodbine Group, East Texas field: Regional and local perspectives

An analysis of 31 whole cores (1600 ft, 490 m) and closely spaced wireline logs (500 wells) penetrating the Lower Cretaceous (Cenomanian) lower Woodbine Group in the mature East Texas field and adjacent areas indicates that depositional origins and complexity of the sandstone-body architecture in the field vary from those inferred from previous studies. Heterogeneity in the lower Woodbine Group is controlled by highstand, fluvial-dominated deltaic depositional architecture, with dip-elongate distributary-channel sandstones pinching out over short distances (typically 500 ft [150 m]) into delta-plain and interdistributary-bay siltstones and mudstones. This highstand section is truncated in the north and west parts of the field by a thick (maximum of 140 ft [43 m]) lowstand, incised-valley-fill succession composed of multistoried, coarse-gravel conglomerate and coarse sandstone beds of bed-load fluvial systems. In some areas of the field, this valley fill directly overlies distal-delta-front deposits, recording a fall in relative sea level of at least 215 ft (65 m). Correlation with the Woodbine succession in the East Texas Basin indicates that these highstand and lowstand deposits occur in the basal three fourth-order sequences of the unit, which comprises a maximum of 14 such cycles. Previous studies of the Woodbine Group have inferred meanderbelt sandstones flanked by coeval flood-plain mudstones and well-connected, laterally continuous sheet sandstones of wave-dominated deltaic and barrier-strand-plain settings. This model is inappropriate, and a full assessment of reservoir compartmentalization, fluid flow, and unswept mobile oil in East Texas field should include the highstand, fluvial-dominated deltaic and lowstand valley-fill sandstone-body architecture.

Seismic sedimentology and regional depositional systems in Mioceno Norte, Lake Maracaibo, Venezuela

Seismic sedimentology is the use of seismic data to study sedimentary rocks and the processes by which they form. It differs from classic seismic stratigraphy in that it uses mainly the horizontal, instead of the vertical, character of seismic data to provide high-resolution images of seismic attribute patterns that can be related to geomorphology and depositional models. A display of seismic attributes on geologic time surfaces is the basic tool of seismic sedimentology. However, there are strict limitations and conditions under which reservoir geometries can be optimally delineated on time and/or horizon slices. Specifically, the slice must be picked at or parallel to a geologic time-equivalent seismic event. This condition is seldom satisfied in practice because of structural dipping and lateral thickness variations of depositional sequences. Stratal slicing of 3-D seismic data overcomes some shortcomings of time slicing and horizon slicing by proportionally slicing between geologic time-equivalent seismic reference events. This facilitates the seismic mapping of sedimentary features on depositional surfaces. Miocene deposits in Mioceno Norte, Lake Maracaibo, Venezuela, provide an excellent example of this seismic sedimentological approach. A high-resolution stratigraphic framework, based on wireline logs, defines approximately 30 genetic stratigraphic units (GSU) separated by marine flooding surfaces, lacustrine flooding surfaces, paleosols, and floodplain shales (Figure 1). These GSUs, each 50–150 ft thick, were correlated by tracing low-resistivity shale markers on logs. The origin, scale, and hierarchy of GSUs in Mioceno Norte are similar to those of depositional episodes and genetic depositional sequences defined by Galloway (1989). A GSU, typically bounded by flooding surfaces or paleosols, represents a cycle of coastal progradation commonly capped by transgressive facies and a condensed section. Detailed isopach and facies mapping based on conventional core and SP/GR log-facies patterns showed that the GSU origins are shoreface, deltaic, and fluvial, respectively. Figure 1. Stratigraphic framework in Mioceno Norte …

Neogene tectonic, stratigraphic, and play framework of the southern Laguna Madre–Tuxpan continental shelf, Gulf of Mexico

Neogene shelf, slope, canyon, and slope-to-basin-floor transition plays in the southern Laguna Madre–Tuxpan (LM-T) continental shelf reflect a variety of structural and stratigraphic controls, including gravity sliding and extension, compression, salt evacuation, and lowstand canyon and fan systems. The Neogene in the LM-T area was deposited along narrow shelves associated with a tectonically active coast affected by significant uplift and erosion of carbonate and volcanic terrains. This study characterizes 4 structurally defined trends and 32 Neogene plays in a more than 50,000-km2 (19,300-mi2) area linking the Veracruz and Burgos basins. The Caonero trend in the southern part of the LM-T area contains deep-seated basement faults caused by Laramide compression. Many of these faults are directly linked to the interpreted Mesozoic source rocks, providing potential pathways for vertically migrating hydrocarbons. In contrast, the Lankahuasa trend, north of the Caonero trend, contains listric faults, which detach into a shallow horizon. This trend is associated with thick Pliocene shelf depocenters. The dominant plays in the Faja de Oro–Nyade trend in the central part of the LM-T area contain thick lower and middle Miocene successions of steeply dipping slope deposits, reflecting significant uplift and erosion of the carbonate Tuxpan platform. These slope plays consist of narrow channel-fill and levee sandstones encased in siltstones and mudstones. Plays in the north end of the LM-T area, in the southern part of the Burgos basin, contain intensely deformed strata linked to salt and shale diapirism. Outer-shelf, slope, and proximal basin-floor plays in the Lamprea trend are internally complex and contain muddy debris-flow and slump deposits. Risk factors and the relative importance of play elements vary greatly among LM-T plays. Reservoir quality is a critical limiting play element in many plays, especially those in the Caonero trend directly downdip from the trans-Mexican volcanic belt, as well as carbonate-rich slope plays adjacent to the Tuxpan platform. In contrast, trap and source are low-risk play elements in the LM-T area because of the abundance of large three-way and four-way closures and the widespread distribution of organic-rich Upper Jurassic Tithonian-age source rock. The potential for hydrocarbon migration in LM-T plays is a function of the distribution of deep-seated faults inferred to intersect the primary Mesozoic source. Their distribution is problematic for the Lankahuasa trend, where listric faults sole out into the Paleocene. Seal is poorly documented for LM-T plays, although the presence of overpressured zones and thick bathyal shales is favorable for seal development in middle and lower Miocene basin and slope plays.

Geologic framework of upper Miocene and Pliocene gas plays of the Macuspana Basin, southeastern Mexico

This integrated study provides a geological and geochemical framework for upper Miocene and Pliocene siliciclastic gas plays in the Macuspana Basin. Structural controls for the plays are deep-seated faults that tap Mesozoic thermogenic gas sources, areas of intense shale diapirism and folding, and areas with structural inversion that could enhance trapping and reservoir productivity. Early Neogene thrusting south of the basin triggered evacuation of Oligocene shale along northwest-dipping listric faults in the eastern and southeastern basin margin. These faults are associated with large-scale rollover structures and thick (500 m) upper Miocene shoreface and wave-dominated deltaic complexes. A second phase of extension in the early Pliocene formed a set of broad, southeast-dipping listric faults in the western basin, controlling thick accumulations of stacked Pliocene shoreface deposits. Trap formation and enhancement in the southern basin margin are linked to late Miocene to Pliocene inversion.The primary stratigraphic controls on play occurrence in the upper Miocene in the onshore part of the basin are the regional facies distribution of northwest-prograding shoreface and wave-dominated deltaic systems. There was a shift in Pliocene sedimentation from the southeast to the west and northwest parts of the basin, where thick successions of aggradational shoreface and wave-dominated deltaic deposits accumulated in depocenters defined by shale evacuation along growth faults. Valley-fill deposits in both the upper Miocene and Pliocene resulted from shortlived periods of base-level change induced by either uplift on the southern basin margin or eustasy. The offshore part of the basin is inferred to consist of deep-water turbidite deposits that formed downdip (westward) of a hypothesized mixed clastic-carbonate prograding complex from the Yucatan platform.Three petroleum systems (Mesozoic, Paleogene–lower Neogene, and upper Miocene–Pliocene) contributed to the hydrocarbon accumulations and hydrocarbon generation and migration in the basin. Principal Upper Jurassic/Lower Cretaceous source rocks generated wet thermogenic gases and oil. Secondary lower Tertiary source rocks generated dominantly dry biogenic gases. Mixtures of the two gas types are common. Numerous deep-seated growth faults and faults serve as pathways for Mesozoic-sourced hydrocarbons. Surface seeps and abundant gas shows suggest that hydrocarbons are being generated today.

Geologic controls on transgressive-regressive cycles in the upper Pictured Cliffs Sandstone and coal geometry in the lower Fruitland Formation, northern San Juan Basin, New Mexico and Colorado

Three upper Pictured Cliffs Sandstone tongues in the northern part of the San Juan Basin record high-frequency transgressive episodes during the Late Cretaceous and are inferred to have been caused by eustatic sea level rise coincident with differential subsidence. Outcrop and subsurface studies show that each tongue is an amalgamated barrier strand-plain unit up to 100 ft (30 m) thick. Successive upper Pictured Cliffs tongues display an imbricate relation and are offset basinward, reflecting net shoreline progradation northeastward. Upper Pictured Cliffs barrier strand-plain sandstones underlie and bound thickest Fruitland coal seams on the seaward side. Controls on Fruitland coal-seam thickness and continuity are a function of local facies distribution in a coastal-plain setting, shoreline positions related to transgressive-regressive cycles, and basin subsidence. During periods of relative sea level rise, the Pictured Cliffs shoreline was temporarily stabilized, allowing thick, coastal-plain peats to accumulate. Although some coal seams in the lower Fruitland tongue override abandoned Pictured Cliffs shoreline deposits, many pinch out against them. Differences in the degree of continuity of these coal seams relative to coeval shoreline sandstones are attributed to either differential subsidence in the northern part of the basin, multiple episodes of sea level rise, local variations in accommodation and progradation, stabilization of the shoreline by aggrading peat deposits, or a combination of these factors. Fruitland coalbed methane resources and productivity are partly controlled by coal-seam thickness; other important factors include thermal maturity, fracturing, and overpressuring. The dominant production trend occurs in the northern part of the basin and is oriented northwestward, coinciding with the greatest Fruitland net coal thickness. Similar relationships between trends of thick coal seams of coeval origin with stacked shoreface sandstones exist in other Western Interior basins in the United States and serve as models for coalbed methane exploration in other basins of the world.

Integration of GPR with stratigraphic and lidar data to investigate behind-the-outcrop 3D geometry of a tidal channel reservoir analog, upper Ferron Sandstone, Utah

Highly compartmentalized reservoirs are commonly attributed to heterogeneous facies as well as vertical/horizontal stratigraphic trends associated with depositional cycles. Detailed information on stratigraphic heterogeneity is essential to fully exploit a reservoir. Subseismic-scale reservoir stratigraphy can be investigated in outcrop by integrating multiple high-resolution sedimentological data, lidar (light detection and ranging) data, and GPR (ground-penetrating radar) data. This combination of different data sets can help to characterize ancient tidal channels exposed in outcrops, which are considered to be heterogeneous compartmentalized sandstone reservoir analogs. This multidisciplinary approach not only attempts to describe rarely seen Cretaceous tidal-channel deposits at the Dry Wash outcrop (Figure 1) in three dimensions, but it also demonstrates how the different types of data can be integrated to better understand ancient complex shallow-marine depositional systems.

Eaglebine play of the southwestern East Texas basin: Stratigraphic and depositional framework of the Upper Cretaceous (Cenomanian–Turonian) Woodbine and Eagle Ford Groups

The Woodbine and Eagle Ford Groups of the southwestern East Texas basin compose an emerging play, which has generated considerable interest because of its potential for new hydrocarbon production from both sandstone and mudrock reservoirs. However, the play’s stratigraphic and depositional relations are complex and directly relate to the play’s exploration challenges. Productive Woodbine and Eagle Ford (sub-Clarksville) sandstones intertongue with a poorly defined, subregional mudrock-dominated interval that thins southwestward toward the San Marcos arch. We propose dividing this succession into two intervals: (1) the Lower unit, a high-gamma-ray unit at the base of this mudrock succession that is inferred to be equivalent to the Maness Shale of the Washita Group and to part of the lower Eagle Ford Group on the San Marcos arch, and (2) an Upper unit, a basinward-thickening zone of consistently lower gamma-ray-log facies inferred to be equivalent to the Woodbine Group, Pepper Shale, and the Eagle Ford Group of the East Texas basin. Because the Cenomanian–Turonian boundary occurs within the Eagle Ford Group of the East Texas basin and the lower Eagle Ford section of the San Marcos arch, most of the Maness-through-Eagle Ford succession exists as a much-thinned section on the arch. Basinwide integration of the Woodbine sequence-stratigraphic framework shows that the number of fourth-order sequences in the unit decreases westward from 14 in the basin axis to no more than 9 in the most active part of the Eaglebine play because of their systematic depositional pinch out approaching the western basin margin. The Eagle Ford Group consists of three fourth-order sequences capped by the sub-Clarksville sandstones that accumulated after the major late Cenomanian–early Turonian flooding event recorded by a basinwide transgressive systems tract (TST) at the base of the unit. Depositional systems of the Woodbine Group vary within the study area, even between stratigraphically adjacent systems. On-shelf siliciclastic systems include fluvial-dominated-delta; incised-valley-fill fluvial and nearshore-marine; and wave-dominated-delta deposits.

Reservoir systems of the Pennsylvanian lower Atoka Group (Bend Conglomerate), northern Fort Worth Basin, Texas: High-resolution facies distribution, structural controls on sedimentation, and production trends

This study defines the depositional systems of mature lower Atoka Group reservoirs, structural influence on their sedimentation, and sand-transport patterns at a higher degree of resolution and over a significantly larger part of the play area than previously conducted. The reservoir systems are characterized by pronounced variations in depositional style, even between stratigraphically adjacent systems. They represent a variety of on-shelf siliciclastic depositional facies, including gravelly braided river, fluvial-dominated delta, and low-sinuosity incised river deposits. Penecontemporaneous, high-angle, basement-rooted reverse faults and genetically associated folds of the Mineral Wells–Newark East fault system exerted direct control on the orientation of complex fluvial-channel and delta-distributary sand-transport pathways and the geometry of deltaic depocenters. Multiple contemporaneous source areas, including the Ouachita fold belt to the southeast, the Muenster arch to the northeast, and the south flank of the Red River arch, also contributed to the complexity of sandstone trends in the lower Atoka play area. Bubble maps of normalized per-well first-year production and total cumulative production allow qualitative conclusions regarding geologic controls on production distribution. Most wells with optimal gas production occur within two northwest-trending production fairways that coincide with primary sandstone trends of one or more reservoir systems. Highest per-well oil production exists where lower Atoka reservoir facies occur above oil-prone Barnett Shale source rocks (vitrinite reflectance 1.1% Ro) in the western and northwestern parts of the study area. Widespread fault-bounded, karst-produced sag structures that extend vertically from the source rocks through the lower Atoka Group most likely served as hydrocarbon-migration conduits and formed traps for both oil and gas.

Greenhouse shoreline migration: Wilcox deltas

ABSTRACT In contrast to the high-frequency and high-amplitude sea level changes of icehouse times, eustatic sea level changes in greenhouse times are now generally accepted as significantly lower frequency and amplitude. As a corollary of this, frequency and extent of cross-shelf shoreline transits in greenhouse times are also likely to have been modest by comparison, and it has been suggested that greenhouse deltas may have been docked at the shelf edge for long periods, thus delivering sediment to deep-water areas more frequently. A revisit of upper Paleocene–lower Eocene Wilcox data across south Texas shows repeated regressive–transgressive shoreline migrations longer than 50 km (31 mi) at a time scale of some 300 k.y. This style of repeated shoreline transits is documented from well logs and is supported by the repeated presence of transgressive estuarine deposits with strong tidal evidence as interpreted from core. We argue, therefore, that Wilcox paleogeography was more varied than commonly portrayed and that the greenhouse shoreline transits were caused by greenhouse sea level change but severely modulated by variable sediment discharge caused by Paleogene hyperthermals. Periodic climate warming during Wilcox deposition and Laramide relief generation in the drainage areas were also responsible for unusually high sediment flux into the Gulf of Mexico. The factor of sediment supply in shoreline growth and retreat has been understated in the literature, partly because of an overemphasis on accommodation as the main driver of stratigraphic sequences.

Reservoir systems of the pennsylvanian lower Atoka Group (Bend Conglomerate), northern Fort Worth Basin, Texas

This study defines the depositional systems of mature lower Atoka Group reservoirs, structural influence on their sedimentation, and sand-transport patterns at a higher degree of resolution and over a significantly larger part of the play area than previously conducted. The reservoir systems are characterized by pronounced variations in depositional style, even between stratigraphically adjacent systems. They represent a variety of on-shelf siliciclastic depositional facies, including gravelly braided river, fluvial-dominated delta, and low-sinuosity incised river deposits. Penecontemporaneous, high-angle, basement-rooted reverse faults and genetically associated folds of the Mineral Wells–Newark East fault system exerted direct control on the orientation of complex fluvial-channel and delta-distributary sand-transport pathways and the geometry of deltaic depocenters. Multiple contemporaneous source areas, including the Ouachita fold belt to the southeast, the Muenster arch to the northeast, and the south flank of the Red River arch, also contributed to the complexity of sandstone trends in the lower Atoka play area. Bubble maps of normalized per-well first-year production and total cumulative production allow qualitative conclusions regarding geologic controls on production distribution. Most wells with optimal gas production occur within two northwest-trending production fairways that coincide with primary sandstone trends of one or more reservoir systems. Highest per-well oil production exists where lower Atoka reservoir facies occur above oil-prone Barnett Shale source rocks (vitrinite reflectance 1.1% Ro) in the western and northwestern parts of the study area. Widespread fault-bounded, karst-produced sag structures that extend vertically from the source rocks through the lower Atoka Group most likely served as hydrocarbon-migration conduits and formed traps for both oil and gas.