L. F. Brown

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.

Depositional Systems and Shelf-Slope Relations on Cratonic Basin Margin, Uppermost Pennsylvanian of North-Central Texas

The Eastern shelf was a constructional platform developed on the margin of the sediment-starved Midland basin during Late Pennsylvanian and Early Permian times. A mixed terrigenous-carbonate sedimentary province characterized the shelf during most of its history. Sediments were derived from highlands on the east and northeast. Along the outcrop belt in north-central Texas, uppermost Pennsylvanian beds comprise the Harpersville Formation, a boundary-defined rock stratigraphic unit within the Cisco Group. Harpersville facies extend westward into the subsurface 50-60 mi (80-96 km), where they grade into equivalent shelf-margin carbonate and slope terrigenous facies. Preserved relief between the shelf margin and basin floor ranges from 600 to 1,100 ft (180-330 m) with dips ra ging up to 5 degrees. Three component depositional systems, recognized on the basis of gross lithologic composition and position relative to the shelf edges, are the Cisco fluvial-deltaic system, the Sylvester shelf-edge bank system, and the Sweetwater slope system. The Cisco fluvial-deltaic system is composed of fluvial-deltaic facies and associated interdeltaic embayment facies. Eight deltaic lobes have been mapped. The Sweetwater slope system is composed of several slope wedges, or fans, each of which includes shelf-margin, slope-trough, and distal-slope sandstone facies, as well as slope mudstone facies. Terrigenous sediments were transported across the shelf by prograding fluvial-deltaic channels that locally extended through the Sylvester shelf-edge bank system and onto the slope where deposition in he deeper basin constructed submarine fans. The Eastern shelf prograded into the Midland basin through contemporaneous, local upbuilding by fluvial, deltaic, and shelf-edge bank deposition and outbuilding by slope-fan deposition. Sites of shelf construction and outbuilding shifted through time in response to sedimentary and structurally controlled abandonment of major delta lobes. Extrabasinal controls such as eustatic sea-level changes were of secondary importance in developing the depositional fabric of the Harpersville shelf sequence. Petroleum is found in all three depositional systems. Principal productive facies include fluvial, distributary-channel, and distributary-mouth bar sandstones of the fluvial-deltaic system and distal slope and shelf-margin sandstones of the slope system. The complex, lenticular geometry of these thin deltaic sandstones affords maximum opportunity for development of stratigraphic and combination traps. Coal beds, consisting of detrital plant debris, are present within inter-deltaic embayment facies.

Clay mineralogy in relation to deltaic sedimentation patterns of Desmoinesian cyclothems in Iowa-Missouri

Almost four decades of study of Desmoinesian strata of Middle Pennsylvanian age in south-central Iowa and north-central Missouri have provided the stratigraphic control required to test the variation of clay mineralogy vertically and laterally within various paralic clay and shale facies. Local and regional variations in clay mineralogy within Desmoinesian strata are generally predictable and are in agreement with current knowledge of deltaic deposition. A principal environmental variation within a deltaic system is the change from normal marine salinities in deltaic marine environments to brackish- and fresh-water conditions in the marshy delta plains, in upper interdistributary bays, and within flanking interdeltaic embayments. Changes from marine to nonmarine facies coincide with a decrease in illite, and an increase in kaolin, mixed-layer clays, and in the percentage of expansible layers in the mixed-layer clay. The principal clay detritus entering the area was illite, which underwent various degrees of alteration in different aqueous and subaerial environments within deltaic and interdeltaic areas. Clay-mineral composition alone does not provide unique environmental answers. The distribution of clay-mineral suites within these systems, however, both supports the deltaic-interdeltaic depositional model and can be understood within the context of this framework.

Evolving sequence-stratigraphic concepts: Emphasis on siliciclastic systems tracts

During the past five years, rapidly evolving new sequence-stratigraphic concepts have begun to impose significant changes in the application of stratigraphy in petroleum exploration and reservoir development. The growing number and variety of oral and published papers on sequence stratigraphy clearly document this stratigraphic revolution. Not since depositional systems concepts evolved in the 1960s to give rise to seismic stratigraphy in the 1970s has the field of stratigraphy changed so rapidly and so fundamentally. Within the next few years, sequence stratigraphy is destined to play an increasingly important role in the way basins are analyzed, hydrocarbon-play potential is assessed, and prospect and production strategies are devised. Geoscientists worldwide are struggling to keep up with the significance of all of these new ideas and techniques.

Re-exploration of cratonic basins using passive-margin sequence-stratigraphic concepts: examples from upper Paleozoic rocks, eastern margin, Midland basin

Use of 5000 well logs and extensive outcrop information with a 22,000-mi/sup 2/ test region on the eastern margin of the Midland basin permitted delineation of 16 probably third-order type 1 depositional sequences. Sandstone-isolith maps of siliciclastic highstand and lowstand systems tracts show that most structural traps produce from highstand fluvial-deltaic reservoirs, but most stratigraphic traps discovered to date occur within lowstand depositional systems, principally incised valley fills and basin-floor fans. Hydrocarbons are rarely trapped in retrogradational (transgressive) systems tracts. Maps of lowstand tracts refocus attention on reservoirs that can be predicted to exist basinward of preexisting shelf edges. A basinward shift of exploration emphasis from incised valley-fill reservoirs to other lowstand elements - such as basin-floor fans, canyon and leveed-channel fills, and lowstand progradational deltaic wedges - could lead to plays where lenticular reservoir sandstones and marine-condensed source and seal shales exhibit the optimum conditions for pinch-out traps.

Recognition of Fluvial and Deltaic Sandstones of Pennsylvanian and Permian Ages in North-Central Texas: ABSTRACT

Sedimentary and stratigraphic evidence indicates that most Virgil and Wolfcamp elongate sandstones on the Eastern shelf of north-central Texas are segments of dip-fed fluvial and deltaic depositional systems. These sandstone bodies are composed of superposed delta front, channel-mouth bars, and distributary channels, on top of which are superimposed fluvial and peripheral sheet and small barlike bodies. Fluvial facies consist of channel-fill sandstones and conglomerates, and overbank mudstones and siltstones; levee deposits are difficult to recognize. Elongate sandstones enclosed in overbank mudstone become finer upward, and characterize fine-grained meander-belt deposits; braided and coarse-grained meander-belt sandstones are extensive tabular to highly belted bodies with little mudstone. Constructional deltaic sandstones become coarser upward. Delta-front facies display parallel and ripple bedforms, and commonly show distorted basal bedding resulting from subsidence into prodeltaic muds. Channel-mouth bars are normally distorted sandstones with trough crossbedding and small scour channels. Symmetrically filled distributary channels are shallow and up to 50 yd wide. Delta-front sandstones grade laterally into thin, destructive sheet sandstones and strandplain facies within adjacent interdeltaic areas. Delta progradation, fluvial aggradation, compaction, avulsion, destruction, and marine transgression, followed by later reoccupation of deltaic and fluvial sites, result in distinctive lateral and vertical facies relations. End_of_Article - Last_Page 332------------