Paul Weimer

Seismic facies and sedimentary processes of submarine fans and turbidite systems

Part I: Techniques and Topics in Turbidite Research. Part II: Seismic Facies and Sedimentary Processes of Ancient Submarine Fans and Turbidite Systems. Part III: Seismic Facies and Sedimentary Processes of Modern Submarine Fans and Turbidite Systems. Appendix 1: Abstracts.

Sequence Stratigraphy, Facies Geometries, and Depositional History of the Mississippi Fan, Gulf of Mexico (1)

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick that was deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19000 km of multifold seismic data across the fan defined 17 seismic sequences, each characterized by a series of channel, levee, and associated overbank deposits, along with other mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10-20% of the sediments in the sequence. These facies are externally mounded in cross section and occur in two general regions of the fan. In the upper and middle fan, they occur below channels and are elongated in shape, mimicking the channels distr bution. In the middle to lower fan, they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites, and are informally called mass transport complexes. Overlying this basal interval and characteristic of all sequences are well-developed channel-levee systems, which constitute 80-90% of the fans sediments. Channels consist of high-amplitude, subparallel reflections. Levee sediments have subparallel reflections that have moderate to high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in the grain size and bed thickness of the levee sediments. Overbank sediments consist of interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites derived from the channel, as well as slides and debris flows derived from the slope. Pliocene-Pleistocene eustatic cycles are interpreted to have been the major factor controlling the timing and style of sedimentation in the fan. Mass transport complexes are interpreted to have formed during a lowering of sea level, and reflect sediments derived from retrogressive slumping during the formation of submarine canyons in the upper slope and outer shelf. Channel-levee systems were deposited when sea level was near its lowest position and sediment derived from deltas was transported into the deep basin via submarine canyons. During highstands in sea level, a thin layer of hemipelagic sediment was deposited on the fan surface. The Mississippi Fan serves as an exploration model for mud-dominated submarine fans and has four prospective reservoir facies: channel sands with linear trends, unchannelized sands beyond the downdip terminus of the channel (possible lobes), potentially sand-prone levees immediately adjacent to initial channels deposited in some sequences, and limited parts of mass transport complexes.

The Perdido fold belt, northwestern deep Gulf of Mexico; Part 1, Structural geometry, evolution and regional implications

The Perdido fold belt is a frontier petroleum exploration province located in deep waters of the northwestern Gulf of Mexico. The anticlines are northeast-southwest-trending, symmetrical to asymmetrical, with concentric folds usually bounded on both flanks by steep reverse faults. The folds are interpreted as detachment folds cored by autochthonous Middle Jurassic Louann Salt. The fold belt overlies rifted transitional crust characterized by northeast-southwest-trending basement highs and northwest-southeast transverse structures that controlled the original salt thickness and subsequent fold geometry. Upper Jurassic-Eocene strata were folded during the early Oligocene (36-30 Ma), with deformation possibly continuing into the earliest Miocene. Postkinematic sediments gradually buried the folds, with younger strata progressively onlapping the highest structures. Some folds were reactivated during the middle Miocene, and a late phase of broad uplift during the Pliocene-Pleistocene is attributed to loading of the Louann Salt by the advancing Sigsbee salt nappe. The Perdido fold belt marks the basinward margin of a complex, linked system of gravitational spreading above salt. Updip Paleogene sedimentary loading and associated extension were accommodated downdip primarily by salt canopy extrusion. The 5-10% shortening and folding occurred only after canopy feeders were evacuated and closed off. Subsequent loading and deformation were concentrated at higher, allochthonous levels.

Sequence stratigraphy of the Mississippi Fan (Plio-Pleistocene), Gulf of Mexico

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick, deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19,000 km of multifold seismic data defined 17 seismic sequences, each characterized by channel, levee, and associated overbank deposits, as well as mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10–20% of the sediments in each the sequences. These facies are externally mounded and occur in two general regions of the fan: (1) in the upper and middle fan they are elongate in shape and mimic the channels distribution; (2) in the middle fan to lower fan they are characterized by a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites (informally called “mass transport complexes”).Overlying this basal interval, characteristic of all sequences, are well-developed channel-levee systems that constitute 80–90% of the fans sediments. Channels consist of high amplitude, subparallel reflections, whereas the flanking levee sediments appear as subparallel reflections that have high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in grain size and bed thicknesses. Overbank sediments are characterized by interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites from the channel, as well as slides and debris flows derived both locally and from the slope updip.

Worlwide deepwater exploration and production Past, present, and future

Approximately 58 billion barrels of oil equivalent have been discovered in deepwater (defined as waters of at least 500 m) from 18 basins on 6 continents (Figure 1). This consists of 39 million barrels of oil and condensate and approximately 112 trillion ft3 of gas. More than half has been discovered since 1995; however, only about 25% of the discoveries are developed or under development and less than 5% have been produced, indicating that large expenditures remain to be made. Most resources have been found in the Gulf of Mexico, Brazil, and West Africa (see map in Pettingill and Weimer, 2001).

Salt-Sediment Interaction, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

Structural and sequence stratigraphic interpretations of two-dimensional seismic and well data from northern Green Canyon and Ewing Bank were integrated to evaluate how salt deformation influenced the distribution of Pliocene-Pleistocene facies in time and space. Two techniques were employed. First, twelve palinspastic maps of near-surface structure were constructed. These were combined with maps of interpreted depositional environments to show how shallow salt diapirism created bathymetric relief that influenced the configuration of sediment transport systems and depocenters through time. Second, tectonostratigraphic packages comprising multiple sequences were defined based on external geometry. Different stacking patterns of these packages characterize four types of minibasins, each with a distinct history of salt evacuation from underlying salt stocks and sheets. Interpreted seismic facies were analyzed within this minibasin framework to evaluate how deep-salt withdrawal influenced the distribution of depositional systems. The results show that both structural and sedimentological variables influenced lithofacies development. External factors dictated the volume and type of systemwide clastic input. Regional factors, such as nearby salt structures and the position of deltas, controlled the dispersal of clastics. Local factors, such as the thickness of underlying salt, influenced minibasin-specific evolution. These factors interacted at three scales: (1) a broad transition from sand-rich ponded settings to shale-dominated bypass settings during the Pliocene-Pleistocene, (2) fluctuations over periods of several sequences that created highly variable stratigraphic stacking patterns, and (3) a progression from ponded to bypass facies within individual sea level cycles. Analysis of these various factors can improve the prediction of reservoir distribution within slope minibasins, and thereby reduce the risk in subsalt and deep-water exploration.

Global Petroleum Occurrences in Submarine Fans and Turbidite Systems

Submarine fans and turbidite systems are major petroleum reservoirs in many sedimentary basins worldwide. More than 80 sedimentary basins contain major petroleum-producing submarine fan deposits, and these reservoirs produce from a variety of structural, stratigraphic, and combined traps. To characterize these reservoir occurrences, tables were constructed for each continent and basin showing reservoir age, formation name(s), tectonic setting, and a field example, if appropriate. In addition, 23 major turbidite reservoirs from eight basins are discussed to illustrate the variability in these kinds of reservoirs. Turbidite reservoirs occur primarily as submarine fan deposits, with some occurrences in debris aprons, canyon-related features, and carbonate deposits of mass transport origin.

The Perdido Fold Belt, Northwestern Deep Gulf of Mexico, Part 2: Seismic Stratigraphy and Petroleum Systems

Analysis of 12,000 km of two-dimensional multifold seismic data shows a thick succession of Mesozoic and Cenozoic deep-water strata in the Perdido fold belt, northwestern deep Gulf of Mexico. These strata differ in seismic facies, areal distribution, and reservoir/petroleum potential. Mesozoic strata are interpreted as dominantly fine-grained carbonates and show minor thickness changes. Cenozoic strata are largely mud-dominated siliciclastic turbidite deposits and vary considerably in thickness across the fold belt. These changes reflect the shifting position of Cenozoic marginal-marine depocenters. Mesozoic reservoir potential consists of fractured Upper Jurassic and Cretaceous deep-water carbonates. Cenozoic reservoir potential consists of siliciclastic deep-water turbidites. Portions of the Paleocene to lower Eocene strata are sand-prone and are the downdip equivalents of the lower and upper Wilcox shallow-marine depocenters. These strata are all incorporated within the folds. Lower to middle Oligocene strata coincide with the main growth phase of the fold belt. Potentially sand-prone middle Oligocene to lower Miocene strata are the downdip equivalents of the Vicksburg (early Oligocene), Frio (Oligocene), and Oakville (early Miocene) shallow-water depocenters. These strata form potential stratigraphic traps against the folds. Mesozoic source potential was modeled assuming Oxfordian, Tithonian, Barremian, and Turonian source beds. One-dimensional thermal maturation modeling showed these sources reached peak oil generation between 51 and 39 Ma, 39 and 8 Ma, 32 and 2 Ma, and 26 and 8 Ma, respectively. Cenozoic source potential was modeled using an Eocene source. Modeling showed this source reached only early oil generation in the basinward half of the fold belt. Thermal maturation was reached by source beds at different times in different locations due to changes in burial depth, amount of structural uplift, and underlying thickness of autochthonous salt. All of these factors indicate that seal and reservoir carry significant risk, but that the potential exists for large petroleum accumulations.

Regional sequence stratigraphic setting and reservoir geology of Morrow incised-valley sandstones (lower Pennsylvanian), eastern Colorado and western Kansas

Oil and gas exploration for the lower Pennsylvanian Morrow Formation of eastern Colorado, western Kansas, and northwestern Oklahoma provides a subsurface data set that transects the entire range of lowstand depositional systems from incised-valley-fill systems to deep-water basin-floor systems in one composite depositional sequence. One compound incised-valley fill that is a part of this system contains three facies tracts with unique reservoir characteristics: (1) the updip facies tract is dominated by amalgamated fluvial channel sandstones, (2) the transition facies tract consists of fluvial channel sandstones interbedded with finer grained estuarine sandstones, and (3) the downdip facies tract consists of ribbonlike fluvial channel sandstones isolated in estuarine shale.A 175-mi-long (283-km-long) longitudinal cross section through one trunk of the incised-valley-fill drainage shows that internal valley-fill strata change significantly as a function of the interplay of varying depositional systems down gradient in the valley. Key contrasts in reservoir performance are documented as a function of changes in reservoir characteristics, trap controls, and trap configurations from updip to downdip in this valley-fill drainage.The strata of the Morrow Formation were deposited in a cratonic basin during a period in the Earths history when the climate was cooler than today. High-frequency changes of sea level across an extremely low-gradient depositional surface controlled erosion and deposition. These facies tracts reflect the response of valley-fill sedimentary processes to high-frequency relative sea level changes resulting from glacio-eustasy. The resultant valley-fill systems have many characteristics in common with published valley-fill models, but have significant differences as well.

Petroleum systems of deepwater settings

This course provides the working geophysicist with a broad overview of the petroleum systems of deepwater settings. The six main elements of petroleum systems will be covered: reservoirs, traps, seals, source rocks, generation, migration, and timing. The course is designed to teach students approximately 80% of what is important. For those interested in further study of a specific topic, each chapter has extensive references for the current literature. About 10% of the current cutting-edge information remains proprietary and cannot be included. Deepwater depositional systems are the one type of reservoir system that cannot be easily reached, observed, and studied in the modern environment, in contrast to other sili-ciclastic and carbonate reservoir systems. The study of deepwater systems requires many remote-observation systems, each of which can provide only one view of the entire depositional system. As a consequence, the study and understanding of deepwater depositional systems as reservoirs have lagged behind those of the other reservoir systems, whose modern processes are more easily observed and documented. For this reason, geoscientists use an integrated approach, working in interdisciplinary teams with multiple data types (Figure 1-1). The types of data used in the study of deep-water deposits include detailed outcrop studies, 2D and 3D seismic-reflection data (both for shallow and deep resolution), cores, log suites, and biostratigraphy. These data sets are routinely incorporated into computer reservoir modeling and simulation (Figure 1-1). The following chapters integrate all of these data types and disciplines to characterize the many facets of deepwater systems. Technologies for deepwater exploration and development are improving rapidly. The intent of the course is to provide information that will be usable even as the technologies advance beyond what we present here. With that in mind, this chapter introduces basic deepwater terminology and concepts for deepwater systems that will be used throughout this book. Geoscientists routinely use several terms to describe the sedimentary processes and characteristics of deepwater settings and deposits. For the sake of consistency in this book, we define these terms as follows. The term deep water is used informally in industry in two ways. First, deep water refers to sediments deposited in water depths considered to be “deep,” i.e., those under gravity-flow processes and located somewhere in the upper- to middle-slope region of a basin. Sediment gravity-flow processes are operative in lakes in relatively shallow water and in cratonic basins where water depths may be less than 300 m. Thus, unless stated otherwise, we use the term deepwater systems to refer to marine-sediment gravity-flow processes, environments, and deposits.