Edgar H. Guevara

Regional controls on geomorphology, hydrology, and ecosystem integrity in the Orinoco Delta, Venezuela

Interacting river discharge, tidal oscillation, and tropical rainfall across the 22,000 km2 Orinoco delta plain support diverse fresh and brackish water ecosystems. To develop environmental baseline information for this largely unpopulated region, we evaluate major coastal plain, shallow marine, and river systems of northeastern South America, which serves to identify principal sources and controls of water and sediment flow into, through, and out of the Orinoco Delta. The regional analysis includes a summary of the geology, hydrodynamics, sediment dynamics, and geomorphic characteristics of the Orinoco drainage basin, river, and delta system. Because the Amazon River is a major source of sediment deposited along the Orinoco coast, we summarize Amazon water and sediment input to the northeastern South American littoral zone. We investigate sediment dynamics and geomorphology of the Guiana coast, where marine processes and Holocene history are similar to the Orinoco coast. Major factors controlling Orinoco Delta water and sediment dynamics include the pronounced annual flood discharge; the uneven distribution of water and sediment discharge across the delta plain; discharge of large volumes of water with low sediment concentrations through the Rio Grande and Araguao distributaries; water and sediment dynamics associated with the Guayana littoral current along the northeastern South American coast; inflow of large volumes of Amazon sediment to the Orinoco coast; development of a fresh water plume seaward of Boca Grande; disruption of the Guayana Current by Trinidad, Boca de Serpientes, and Gulf of Paria; and the constriction at Boca de Serpientes.

Holocene evolution of the western Orinoco Delta, Venezuela

The pristine nature of the Orinoco Delta of eastern Venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta. Remote-sensing images, shallow cores, and radiocarbon-dating of organic remains form the basis for describing deltaic environments and interpreting the Holocene history of the delta. The Orinoco Delta can be subdivided into two major sectors. The southeast sector is dominated by the Rio Grande—the principal distributary—and complex networks of anastomosing fluvial and tidal channels. The abundance of siliciclastic deposits suggests that fluvial processes such as overbank flooding strongly influence this part of the delta. In contrast, the northwest sector is represented by few major distributaries, and overbank sedimentation is less widespread relative to the southeast sector. Peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km 2 in area. Northwest-directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast. Mapping of surface sediments, vegetation, and major landforms identified four principal geomorphic systems within the western delta plain: (1) distributary channels, (2) interdistributary flood basins, (3) fluvial-marine transitional environments, and (4) marine-influenced coastal environments. Coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20–30 km during the late Holocene sea-level highstand. Progradation has been accomplished by a combination of distributary avulsion and mudcape progradation. This style of deltaic progradation differs markedly from other deltas such as the Mississippi where distributary avulsion leads to coastal land loss, rather than shoreline progradation. The key difference is that the Orinoco Delta coastal zone receives prodigious amounts of sediment from northwest-moving littoral currents that transport sediment from as far away as the Amazon system (∼1600 km). Late Holocene progradation of the delta has decreased delta-plain gradients, increased water levels, and minimized overbank flooding and siliciclastic sedimentation in the northwest sector. These conditions, coupled with large amounts of direct precipitation, have led to widespread peat accumulation in interdistributary basins. Because peat-forming environments cover up to 5000 km 2 of the delta plain, the Orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits.

Mud volcanoes of the Orinoco Delta, Eastern Venezuela

Abstract Mud volcanoes along the northwest margin of the Orinoco Delta are part of a regional belt of soft sediment deformation and diapirism that formed in response to rapid foredeep sedimentation and subsequent tectonic compression along the Caribbean–South American plate boundary. Field studies of five mud volcanoes show that such structures consist of a central mound covered by active and inactive vents. Inactive vents and mud flows are densely vegetated, whereas active vents are sparsely vegetated. Four out of the five mud volcanoes studied are currently active. Orinoco mud flows consist of mud and clayey silt matrix surrounding lithic clasts of varying composition. Preliminary analysis suggests that the mud volcano sediment is derived from underlying Miocene and Pliocene strata. Hydrocarbon seeps are associated with several of the active mud volcanoes. Orinoco mud volcanoes overlie the crest of a mud-diapir-cored anticline located along the axis of the Eastern Venezuelan Basin. Faulting along the flank of the Pedernales mud volcano suggests that fluidized sediment and hydrocarbons migrate to the surface along faults produced by tensional stresses along the crest of the anticline. Orinoco mud volcanoes highlight the proximity of this major delta to an active plate margin and the importance of tectonic influences on its development. Evaluation of the Orinoco Delta mud volcanoes and those elsewhere indicates that these features are important indicators of compressional tectonism along deformation fronts of plate margins.

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.

Abstract: Outcrop-Constrained Characterization of Stratigraphic Architecture in Deltaic Gas Reservoirs, Lake Creek Unit, Texas

ABSTRACT The Lake Creek Unit of the Houston Embayment, Texas, encompassing 4.32 mi and containing 47 wells that penetrate deltaic sandstones of the Wilcox Group (lower Eocene), offers an exceptional opportunity to assess reservoir heterogeneity in gas reservoirs. Stratigraphic architecture of the G sandstone, 1 of 18 gas-condensate reservoirs ranging in depth from approximately 9,200 to 14,500 ft, was determined using wireline logs and cores. Models of deltaic architecture developed in outcrops of the analogous, deltaic Ferron Sandstone (Cretaceous), Utah, were used to constrain the subsurface interpretation. The G sandstone, approximately 300 ft thick, is one of several upward-coarsening intervals in the lower Wilcox. Mud-rich intervals that separate deltaic intervals and represent local flooding of abandoned, foundering delta lobes allow delineation of four parasequences. Each parasequence comprises genetically related depositional facies showing dimensions and spatial distribution comparable to those in outcrops of the Ferron Sandstone. The youngest parasequence is thinner and contains more abundant, marine-influenced facies than do underlying parasequences, indicating depositional landward stepping similar to that of the upper Ferron sandstones. The upper and lower parts of the parasequences are interdistributary-bay and shelf mud-rich facies, locally lignitic, and crevasse-splay and delta-destructional sandstones. They bound sand-rich intervals comprising delta-front facies locally overlain or replaced by channel-mouth-bar facies, in turn locally overlain or replaced by distributary-channel facies. Permeabilities range from about 20 md to less than 1 md but are mostly less than 1 md, the best values generally corresponding to channel-mouth-bar and distributary-channel sandstones. Deltaic sandstones and intervening bay/shelf shales result in stratified reservoirs. Similar to the Ferron Sandstone, fluid-flow barriers and baffles locally develop because of contrasting permeabilities across facies boundaries within the parasequences and the occurrence of low-permeability, mostly mud-rich intervals interbedded with the deltaic sandstones. Stratigraphic heterogeneity and low-permeability, small-drainage radii locally result in untapped and incompletely drained compartments. End_of_Record - Last_Page 807-------

Influence of Facies Architecture on Hydrocarbon Recovery from Naturally Fractured Submarine Fan Reservoirs, Central Spraberry Trend (Permian), Midland Basin, TX: ABSTRACT

The Spraberry in the central Spraberry trend is a multipay play. The most productive reservoirs are thicker and coarser grained beds capping the upward-coarsening, progradational fan sequences of the upper and lower Spraberry. These naturally fractured, solution gas drive reservoirs have undergone local imbibition waterflooding since 1961. Besides the elusive fracture permeability in these tight reservoirs, the less than 10% recovery is attributed to reservoir heterogeneity. When reservoirs are isolated, hydrocarbon zones are not thoroughly drained by well completions or are affected by fracture treatments or waterflooding. To improve recovery from Spraberry trend fan sequences, strategies must incorporate selective completions for injection and production that consider the facies-controlled reservoir heterogeneities.

Upper Cretaceous-Lower Eocene Strata, Hainesville, Keechi, and Oakwood Salt Domes, East Texas: ABSTRACT

ABSTRACT Salt domes in east Texas are possible sites for nuclear waste repositories. Tectonic stability is a critical factor in evaluating suitability as a repository. Subsurface studies were undertaken to determine stratigraphy and structure of strata around the domes, and to ascertain the growth history of the domes as a means to evaluate tectonic stability. This paper presents the stratigraphy and structural relationships of Upper Cretaceous to lower Eocene strata bounding the upper part of the domes. Three main types of domes have been interpreted from well-log data: 1) at Hainesville dome, Upper Cretaceous strata exhibit notable thickening in a rim syncline and stratigraphic markers dip toward the dome except near the contact with domal material; 2) at Keechi dome, strata are uplifted and dip away from the dome with strata thinning toward the dome; 3) at Oakwood dome, strata are approximately horizontal until near the dome edge, where they are upturned; minor thickening of strata occurs toward the dome. Differences in stratigraphy and structure of Cretaceous-Eocene strata in the vicinity of these domes are attributed to differences in growth history.