Tucker F. Hentz

High-frequency sequence stratigraphy from seismic sedimentology: Applied to Miocene, Vermilion Block 50, Tiger Shoal area, offshore Louisiana

We introduce a seismic-sedimentological approach for mapping high-frequency (fourth-order) sequences and systems tracts using well and three-dimensional seismic data. Key techniques include (1) conditioning seismic data to log lithology by 90 phasing for better well-log integration and (2) imaging and interpreting the sequential, planoform geomorphology of the depositional systems. We recommend a new interpretation procedure that shifts the emphasis of high-frequency sequence-stratigraphic studies from interpreting vertical seismic sections to analyzing more horizontal, high-resolution, seismic-geomorphologic information.This case study shows that stratal slicing in lithology-conditioned seismic data provides sequential seismic imagery of generally contemporaneous depositional systems. This imagery, in turn, serves as a basis for recognizing and mapping high-frequency systems tracts, sequence boundaries, and sequences in a geologic-time domain. In Miocene strata of offshore Louisiana, fourth-order sequences or sequence sets from well data can be seismically mapped at a resolution equivalent to 30 ft (10 m) in thickness from a 30-Hz dominant-frequency seismic data set. The resolution is sufficient for an accurate reconstruction of the high-frequency sequence-stratigraphic framework in the region of seismic coverage outside well control.

Stratal slicing of Miocene-Pliocene sediments in Vermilion Block 50-Tiger Shoal Area, offshore Louisiana

Many people have viewed modern land surfaces from commercial airplanes and marveled at the form and geometry of geomorphic features such as river channels, deltas, barrier islands, and dune fields. These views represent complete images of the modern time surfaces. We can classify the depositional nature of features on these images by interpreting their planiform geometry and geographic context. In fact, modern 3-D seismic technology has made it possible for us to image similar, but much older, geomorphic or depositional features preserved in the rock record. Unfortunately, although many reservoir-scale (well-to-well scale) features can be detected in vertical seismic lines, few such features can be resolved and interpreted in the vertical perspective because of the datas limited bandwidth. Only in the horizontal perspective are such depositional features large enough to be resolvable when displayed in map view on geologic time surfaces.

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.

Early Jurassic sedimentation of a rift-valley lake: Culpeper basin, northern Virginia

Gray, calcareous, lacustrine strata of Early Jurassic age crop out adjacent to the faulted western margin of the Culpeper basin, a Mesozoic graben dominated by fluviogenic red beds. Sedimentological, mineralogical, and paleontological aspects reveal a distinct zonation of littoral, sublittoral, and profundal deposits in the lacustrine sequence. Reconstructed physiographic profiles of the lake exhibit shore-terrace, slope, and basin-plain subenvironments. Dominant sedimentary processes in these domains were, respectively, sediment agitation by waves; deposition from suspension, slumping, and proximal sediment gravity flows; and suspension and distal turbidity current deposition. Deposition of the lacustrine sediments occurred in a eutrophic, calcite-precipitating, permanently or normally stratified lake. The lake occupied an elongate, actively subsiding sub-basin adjacent to, or possibly straddling, the western border fault. The sedimentary fill of the sub-basin is characterized by several cyclic fluvio-lacustrine sequences which average 150 m in thickness and closely resemble sedimentary cycles of other Newark rift basins. The maximum areal extent of the lake may have exceeded 3,000 km2, with an average maximum depth of ∼35 m. Proterozoic and lower Paleozoic strata of the Blue Ridge anticlinorium to the west constituted the major source of clastic sediments in the lacustrine sequence, but uptilted basalt flows and interbedded sedimentary units of the Culpeper Group simultaneously shed detritus from the east.

High-frequency Miocene sequence stratigraphy, offshore Louisiana: Cycle framework and influence on production distribution in a mature shelf province

The regressive Miocene succession of offshore Louisiana comprises 10 third-order sequences and no fewer than 58 fourth-order sequences, which average approximately 1.1 and 0.19 m.y. in duration, respectively, comparable to durations measured in the Gulf Coast Basin and basins worldwide. Upper lower to middle Miocene distal third-order sequences comprise mostly lowstand prograding-wedge, slope-fan, and basin-floor-fan deposits. In contrast, middle to upper Miocene medial sequences record progressively more landward systems tracts: (1) the lateral transition between on-shelf incised-valley fills and the proximal parts of basinward-thickening, lowstand prograding wedges and (2) cyclic on-shelf highstand and transgressive systems tracts. Upper Miocene inner-shelf and marginal marine systems tracts and more abundant incised valleys dominate the thinner proximal third-order sequences.This genetic framework has a major influence on hydrocarbon distribution. Although a strong structural-trapping component is present in the fields, more than 90% of cumulative production originates where fourth-order systems tracts stack to form third-order lowstand systems tracts in all 10 third-order sequences. The development of a high-frequency sequence framework for the prolific Miocene succession and the discovery that hydrocarbons are pooled within the Miocene third-order lowstand systems tracts yield a focused model for the development of abundant undiscovered Miocene reserves in the mature northern Gulf of Mexico shelf province.

Evaporite-hosted native sulfur in Trans-Pecos Texas: Relations to late-phase Basin and Range deformation

Major deposits of biogenic native sulfur are associated with narrow, northeast-trending grabens and normal faults that disrupt the gently tilted, east-dipping Upper Permian evaporite succession of the western Delaware Basin in Trans-Pecos Texas. Orebodies are restricted to geologic traps in the fractured and dissolution-modified downfaulted blocks of the grabens. Other parallel, regionally distributed grabens and normal faults are commonly the sites of noncommercial sulfur deposits and genetically related secondary-replacement (diagenetic) limestone bodies. The sulfur-bearing structures probably formed during the later of two episodes of Basin and Range extension that have not previously been differentiated in Texas but are well defined elsewhere in the western United States. In Texas several lines of evidence collectively support the existence of late-phase, northwest-directed extension that was initiated in the middle Miocene.

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.

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.

Major structural elements of the Miocene section, Burgos Basin, northeastern Mexico

Major Miocene structural elements of the Burgos Basin include a regionwide detachment system that connects extensional fault systems throughout the basin with an active diapir belt downdip, a regionwide pattern of downthrown extensional rollover folds, pervasive secondary faults, and salt and shale diapiric masses in the eastern part of the basin. An interpretation of two-dimensional seismic data suggests that the Burgos Basin Miocene section can be divided into four structural domains: expanded zone, Lamprea trend, Corsair-Wanda trend, and diapir belt. The westernmost unexpanded zone is the footwall of the expanded system part of the basin, which overlies a domain of Oligocene extension. Remaining trends represent an extensional accommodation related to the basinward migration of mobile salt and shale, which has produced a relatively uniform structural style in the Miocene section. The structural style observed in the Burgos Basin appears to define a transitional zone between gravitational collapse in the offshore Laguna Madre-Tuxpan shelf to the south and salt-related raft tectonics of the south Texas Gulf Coast.