Dallas B. Dunlap

Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains

Discovery of giant polygonal terrains on Mars has prompted a 30-year debate over how they formed. The prevailing hypothesis is that small-scale Martian polygons formed by thermal contraction, as in terrestrial permafrost environments. Large-scale (>1 km) Martian polygons in the northern plains are visible in THEMIS, MOLA, Viking, and Mariner data, but how they formed remains enigmatic. We suggest that terrestrial deep-water marine polygons are morphological and perhaps genetic analogs to largescale Martian polygonal features. The terrestrial, deep-water polygons are imaged in three-dimensional seismic-reflection data acquired by the oil and gas industry in offshore Norway and the Gulf of Mexico. How deep-water polygonal fault systems form is a debated topic beyond the scope of this work. However, similarities between terrestrial deep-water polygonal fault systems and large-scale Martian polygonal terrains suggest that the latter could have formed during deep-water marine deposition. Deep-water polygonal faults form within fine-grained sediment at shallow burial depths. Increases in slope angles can trigger downslope disaggregation of deep-water polygons and mass wasting (forming debris flows). Physical models indicate that multidirectional extension can cause polygonal features to break up on a slope over a mobile substrate. Some knobby terrains in the Vastitas Borealis Formation seem to originate from disaggregation of large-scale Martian polygonal terrains. These analogies suggest a possible deep-water subaqueous origin for large-scale Martian polygonal terrains and support the idea of a late Hesperian–early Amazonian ocean on the northern plains of Mars.

Seismic geomorphology of offshore Morocco's east margin, Safi Haute Mer area

The lower continental slope of Moroccos west coast consists of Triassic-age salt manifested in the form of diapirs, tongues, sheets, canopies, and toe thrusts. Active salt diapirism and regional tectonics greatly influence the morphology of the modern sea floor, forming a severely rugose expression with ongoing minibasin development and episodic submarine failure. Detailed mapping of a 1064-km2 (411-mi2) seismic survey acquired in the Safi Haute Mer area revealed that Jurassic to Holocene salt mobilization continually affected distribution of sediment, causing a range of depositional flow styles, from slumps to sheet slides and mass-transport complexes (MTCs). Large sediment waves (20 km [12 mi] long, 1.5-km [0.9-mi] wavelength) were also documented at the end of the Aptian. An east-west–trending structural anticline downdip of the salt activated during initiation of the Atlas uplift in the latest Cretaceous to earliest Tertiary and shaped much of the lower slope into the Tertiary with a persistent canyon system and slope channels. The largest of the debris flows is a Cretaceous-age MTC, a 500-m (1640-ft)-thick flow that spans an area of up to 20,000 km2 (7722 mi2). Composing the MTC are (1) chaotic, mounded seismic facies; (2) internal syndepositional thrusts; and (3) transported megablocks (3.3 km2 [1.3 mi2]) with preserved internal stratigraphy. The MTC originated from an upslope collapse of a narrow shelf during the earliest phases of the Alpine orogeny.

Traps and turbidite reservoir characteristics from a complex and evolving tectonic setting, Veracruz Basin, southeastern Mexico

The Miocene and Pliocene interval of the Veracruz Basin, southeast Mexico, experienced an evolving array of shortening, strike-slip, and volcanic forces in response to plate-scale interactions. The basin is divided into six structural domains that define regions of comparable timing and type of structural deformation, and the basin fill is separated into two long-term depositional phases, each of which can be tied to a waning and then waxing of major basin-bounding tectonic events. The first phase of deposition took place from the early to late Miocene and is tied to the waning effects of the Laramide orogeny. The Miocene basin inherited a tectonically steepened basin margin, across which deep canyons were carved and variably filled with mudstone and thin remnants of coarse sandstone and conglomerate. This zone of erosion and bypass grades into thick, sandstone-rich basin-floor fans. Later in phase I, subaqueous volcanoes, tied to distant plate subduction, developed offshore and formed a bathymetric barrier that prevented turbidity currents from entering the ancestral Gulf of Mexico. The volcanoes also served as immovable buttresses, around which intrabasinal thrust belts developed in response to regional shortening.The second depositional phase is tied to the onset of internal basin shortening and uplift of the north basin margin known as the trans-Mexican volcanic belt. This uplift caused a dramatic reconfiguration of the sediment-dispersal system, whereby large shelf clinoforms prograded from north to south across the basin. In contrast to the onlapping stacking pattern of phase I units, phase II units stack in a strongly offlapping pattern.Proven and postulated reservoir-trap combinations, ranging from four-way to three-way combination (stratigraphic), to pure stratigraphic traps are common. Four-way closures mapped from the two-dimensional and three-dimensional seismic data are large (P50: 5000 km2) and are covered with thick, lower Miocene fan sandstones. Traps that depend on a stratigraphic component are thinner and smaller in size (P50: 1000 km2), but more numerous than the four-way closures. Because many structures have experienced prolonged pulses of compression, top seal is considered an important geologic risk to the retention of substantial gas-column heights.

Shelf-edge deltas along structurally complex margins: A case study from eastern offshore Trinidad

A 15,000-km2 (5792-mi2) three-dimensional seismic data survey that covers the shelf-slope transition of the eastern offshore Trinidad continental margin reveals the geometry and depositional history of the last maximum glacial lowstand shelf-margin succession. Despite the lack of well information at these shallow depths, the quality and continuity of the seismic data allow us to pursue a detailed seismic stratigraphic interpretation of the last lowstand margin succession. The basin-fill stratal architecture of the studied stratigraphic interval shows a great deal of lateral and vertical variability along the continental margin during the Pleistocene to Holocene. Three geomorphological elements controlled the character of the accommodation within the basin and were crucial in transporting, delivering, and storing sediment supply from shelf to slope areas: (1) the Columbus sedimentary pathway on the shelf, (2) bypass zones in the shelf-break region, and (3) deep-water depocenters. The location and geometry of these geomorphological elements within the basin are clearly controlled by underlying structures, transpressional to the north and gravity driven to the south. Migration of the paleo-Orinoco delta system across the shelf was also a key factor in defining the stratigraphic geometries that are observed within the shelf break. Development of shelf-edge versus outer-shelf deltaic systems on the continental margin was controlled by the nature of sediment supply at specific times, as well as by the availability of accommodation, although, to a lesser extent, to relative sea level fluctuations. The interpretation also showed that, for time-equivalent units, parts of the shelf-edge region could develop as an erosional margin (sediment bypass zones), whereas other parts of the shelf edge could behave as an accretionary margin (sediment accumulation). The sequence-stratigraphic interpretation that was attempted in this work also demonstrated that the characteristics of systems tracts can abruptly change along strike in the shelf-edge region for time-equivalent units. These changes can be misleading if a genetic interpretation is pursued only on the basis of the definition of system tracts in the shelf-edge region without the consideration of a complete regional framework.

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.

Seismic geomorphology of early North Atlantic sediment waves, offshore northwest Africa

AbstractThe lower continental rise of Morocco’s Atlantic Margin contains three varying morphologies of buried deepwater sediment waves. The 3D mapping of a 1,064-km2 seismic survey acquired in the Safi Haute Mer seismic block revealed numerous linear features that range from small, less than 17-m-thick Jurassic-age amplitude striations up to 110-m-thick migrating Cretaceous contourite sediment waves. Early proto-Atlantic deposition in Safi Haute Mer initiated in the Triassic, with syn- and postrift accumulation in basement half-grabens basinward of the modern Moroccan salt front. Sedimentation continued through the Mesozoic with deposition of turbidites, progradation of clinoforms, and culminating in multiple Late Cretaceous, regionally expansive, mass-transport deposits (20,000  km2). Tertiary stratigraphy consists of multiple thin, pelagic drapes and unconformities. The complex history of sedimentation and tectonics gave rise to three styles of sediment waves found within the study area: (1) type J1 — s...

Structural and hydrogeologic evolution of the Putumayo basin and adjacent fold-thrust belt, Colombia

This multidisciplinary study evaluates the structural and hydrogeologic evolution of Cretaceous-age reservoirs in the Putumayo basin, Colombia. We focused on the Eastern Cordillera fold-thrust belt along the southern Garzon Massif. Many important hydrocarbon accumulations occurred regionally along the proximal foreland basin and frontal fold-thrust belt defining the eastern margin of the northern Andes. To understand why recent Putumayo basin and adjacent thrust belt exploration has resulted in a wide range of oil quality and limited economic discoveries, we reconstructed the structural evolution, timing of oil migration, and timing of groundwater infiltration by (1) assessing regional trends in formation water, oil, and reservoir properties; (2) quantifying the timing of hydrocarbon generation and migration relative to trap formation using (a) two-dimensional (2-D) and three-dimensional seismic data to define and constrain a restorable balanced cross section from the Upper Magdalena Valley to the Putumayo foreland and (b) coupled one-dimensional thermal basin modeling; (3) evaluating the potential roles of Mesozoic extensional faulting and Paleogene shortening in the generation and preservation of structural traps; and (4) assessing groundwater influx from the modern foothills into the reservoir using a 2-D numerical groundwater flow model. We suggest that four-way closure is limited in the study area, where most foreland-verging structures create three-way fault closures that do not effectively trap light hydrocarbons. In addition, east-dipping structures and a relatively large reservoir outcrop area provide water infiltration pathways. Groundwater modeling suggests reservoirs were water washed by 2–200 million pore volumes since Andean uplift. Finally, average reservoir temperatures are <80°C (<176°F), which further facilitated biodegradation.

Qualities of a good reviewer

Interpretation shares commonalities with Geophysics and the AAPG Bulletin in that it is a peer-reviewed journal. Unlike Geophysics and the AAPG Bulletin , Interpretation is built around special sections headed by a team of special-section editors who are either experts or particularly interested in

Seismic geomorphological analysis and hydrocarbon potential of the Lower Cretaceous Cromer Knoll Group, Heidrun field, Norway

The Heidrun field, located on the Halten Terrace of the mid-Norwegian continental shelf, was one of the first giant oil fields found in the Norwegian Sea. Traditional reservoir intervals in the Heidrun field lie within the Jurassic synrift sequence. Most Norwegian continental shelf fields have been producing from these Jurassic reservoirs for the past 30 yr. Production has since declined in these mature fields, but recently, exploration for new reservoirs has resurged in this region. The Jurassic rifted fault blocks form a narrow continental shelf in Norway, thereby greatly reducing the areal extent for exploration and development within existing fields. As the rift axis is approached farther offshore, these Jurassic reservoirs become very deep, too risky to drill, and uneconomical. This risk has prompted exploration in more recent years of the shallower Cretaceous, postrift stratigraphic succession. Cretaceous turbidites have been found in the Norwegian and North Seas, and the discovery of the Agat field in the Norwegian North Sea confirms the existence of a working petroleum system capable of charging Cretaceous reservoirs. These Cretaceous reservoirs were deposited as slope- and basin-floor fans within a series of underfilled rifted deeps along the Norwegian continental shelf and are thought to be sourced from the localized erosion of Jurassic rifted highs. We use three-dimensional seismic and well data to document the geomorphology of a deep-water, Lower Cretaceous wedge (Cromer Knoll Group) within the hanging wall of a rift-related half graben formed on the Halten Terrace offshore mid-Norway. Seismic attribute extractions taken within this Lower Cretaceous wedge reveal the presence of several lobate to elongated bodies that seem to cascade over fault-bounded terraces associated with rifted structures. These high-amplitude, elongated bodies are interpreted as deep-water sedimentary conduits that are time equivalent to the Cretaceous basin-floor fans in more distal parts of the basin to the west. These half-graben fills have the potential to contain high-quality Cretaceous sandstones that might represent a potential new reservoir interval within the Heidrun field.