Downthrown Block Research Articles

Use of soils and colluvial deposits in analyzing tectonic events — The southern Arava Rift, Israel

Buried soils in colluvium units developed on downthrown fault blocks have been presumed to form during periods of tectonic quiescence. This assumes however, that all colluvial units on a fault scarp are tectonically induced, particularly for a multiple-event fault scarp. To test this hypothesis we compared the colluvial stratigraphy and pattern of soil development on two non-tectonic scarps (terrace risers) and a nearby multiple-event fault scarp on the same alluvial fan surface. Both slopes developed under extremely and climatic conditions. The colluvial stratigraphy and soil catenary relationship developed on terrace risers were found to be substantially different from those on the multiple-event fault scarp. The fault scarp colluvium contained several colluvial units separated by soils, whereas only two colluvial units without buried soils were found on the terrace risers. The colluvial units on the fault scarp were triangular in shape, while those on the terrace riser were concave and overlapped up the slope. Cumulic soils, indicating a continuous and low rate of deposition during pedogenesis, were formed along the terrace riser. In contrast, buried soils indicating episodic deposition occurred on the downthrown block of the multiple-event fault scarp. Accordingly, the colluvial stratigraphy developed on the fault scarp can be viewed as a record of distinct tectonic events with no climatically controlled deposition. The data indicate that soils and sedimentary stratigraphy can serve as useful indicators to distinguish terrace risers and single-event fault scarps from multiple-event fault scarps. These indicators can be helpful in distinguishing phases of tectonic activity and in evaluating recurrence intervals on a multiple-fault scarp.

Surface hydrothermal alteration and evolution of the Te Kopia Thermal Area, New Zealand

The Te Kopia Thermal Area has surface manifestations that extend over an area of about 3 × 1.2 km along the Paeroa Fault Zone. Steaming ground, fumaroles, mud and acid sulphate pools are present on both the upthrown and downthrown blocks of the Paeroa Fault, but near-neutral pH chloride-bicarbonate springs discharge up to 2 km west of the fault trace. The host rocks comprise mainly Quaternary ignimbrites that dip gently (∼7°) eastwards but are vertically displaced by the fault by several hundred metres. Hydrothermal alteration is widespread, and the present thermal activity is now producing kaolinite, alunite, silica residue, hematite and cristobalite. This assemblage occurs both in the otherwise unaltered ignimbrites, as well as overprinting the products of earlier hydrothermal activity. Evidence for changes that have occurred in the nature and extent of thermal activity at Te Kopia include: (1) The widespread overprint of a kaolinite-alunite-cristobalite assemblage upon hydrothermal minerals (including adularia, quartz and mordenite) produced by alkali-chloride, bicarbonate-chloride, and heated groundwaters. (2) The occurrence of silica sinter which deposited from alkali chloride springs that discharged about 3000 years ago. This is now a steam zone. (3) Areas of cold ground that were once hot, as demonstrated by the presence of hydrothermal minerals. (4) The presence of euhedral quartz crystals at the surface on the upthrown fault block. Fluid inclusions in these crystals from two places homogenise at 188 ±15° C and 196 ±11° C. The trapped fluids have apparent salinities between 0.2 and 0.4 wt % NaCl equivalent. The quartz crystals thus grew at depths of at least 120 m below the water table. Vertical movements along the Paeroa Fault totalling at least 300 m have uplifted these crystals to their present positions. The surface geology and alteration provide evidence that thermal activity at Te Kopia has been long lived, possibly as long at 120 000 years. However, the hydrology of the system and its thermal regime have changed greatly during its lifetime, mainly in response to movements along the Paeroa Fault.

Stable isotope study of antimony deposits in the Muratdagi region, western Turkey

The Muratdagi region is rich in antimony deposits having the following common characteristics: post Miocene age, location on the down-thrown blocks next to normal faults, in the vicinity of active or fossil thermal springs, and in contact with carbonate rocks. The isotopic composition of — 7‰. SMOW of the mineralizing fluid calculated from the measured °18O of quartz and the fluid inclusion microthermometry, is indicative of meteoric water origin. The °13C of the inclusion CO2 of — 19.1 to — 25.4‰ PDB is indicative of interaction with organic material-graphite. The °34S of stibnite — 3.6 to — 0.7‰ is, in view of the mineral assemblage, indicative of magmatic origin of the sulphur. A tightly confined set of structural, lithological, hydrological and geochemical features define a sequence of geochemical processes; formation of acid and reducing fluid, leaching and transport of antimony complexes and precipitation of stibnite within defined lithological units. The set of processes seems to have taken place within a space of 5000 m lateral and 1000 m vertical extension.

Kinematics of experimental forced folds and their relevance to cross-section balancing

Abstract We report the results of a series of laboratory experiments in which packages of layers (consisting of rock and lead) are deformed under 50 MPa confining pressure into forced folds by the uplift and rotation of pre-machined steel forcing blocks. The models are not fully scaled, but the geometries resulting form the deformation are remarkably similar to many natural forced folds observed in the U.S. Rocky Mountains foreland. During the folding, detachment and quantifiable layer-parallel translation occur between the layered sequence and the forcing assembly, confirming limited observations from earlier model studies. Away from the fold, there is a pattern of movement in which the layered sequence first moves away from the uplift, but, with greater structural relief, those motions reverse their sense to become layer-parallel translations towards the uplift. The very ductile lead unit at the base of the layered sequence flows laterally, especially across the crest of the uplift to the downthrown block, in response to pressure gradients which are inherent to asymmetric uplifts. The flow of this ductile unit causes layer-parallel translation of the rock layers, thereby transporting material into the fold. If the models were to be treated as proposed cross sections, and if they were subjected to the usual techniques of cross-section balancing, incorrect interpretations would result; this is because there are no suitable sites for either pin lines or no-flow boundaries.

Bedding-plane slip over a pre-existing fault, an example: the Ramon Fault, Israel

Small-scale tectonic structures were found to reveal widely distributed bedding-parallel slip in the sedimentary cover of the central Negev. The orientation of these structures indicates that this region has undergone consistent upper-sheet movements to the south. Bedding-parallel slip zones are situated at the contact between rocks with different mechanical properties. The local extension fields were generated during the subsequent southward bedding-parallel slip above the downthrown block of the Ramon Fault. In the extension zone the normal rock sequence has undergone dislocation by means of bedding-plane faults and oblique faults. The structure development resembles boudinage in macroscale. The southward bedding-plane slip was responsible for compression in the transltion zone from tilted to horizontal bedding. This zone was localized along the synclinal hinge of the tilted block and characterized by development of folds and fault blocks overturned in the transport direction.

Inversion Tectonics in the Maracaibo Area, Venezuela

Sequence stratigraphy and structural interpretation of regional composite seismic lines and new 3D seismic data of Lake Maracaibo lead us to question the old [open quotes]strike-slip fault movement[close quotes] geological paradigm as the major cause of deformation in this area. The pre-Miocene compressional structures of the majority of the oil fields of this area such as Ceuta, Lama, Motatan, etc., are easily and better explained as the result of a Tertiary Tectonic Inversion. Such inversion can be interpreted as a consequence of shortening of [open quotes]depocenters[close quotes] (grabens and half grabens) developed during extensional phases of a back-arc basin. During this shortening, the old faults bordering the [open quotes]depocenters[close quotes] were reactivated with reverse movements creating large compressional structural traps in the hanging wall (old downthrown block). Thin-skinned compressional deformation can occur, locally, outside the major [open quotes]depocenters[close quotes] in the footwall areas between faults of opposite vergence.

Tectonic Boundaries of the Eastern Gulf Coast of North America

Two Precambrian extensional fault episodes, recorded in mapping from central Arkansas across Mississippi, central Alabama, southern Georgia, and into the Atlantic Ocean affected later Pennsylvanian and Triassic tectonics. This interpretation is from magnetic anomaly data and is supported by seismic, gravity, core, and well-log data. The fault system was first suspected from an anomalous magnetic high, representing a feature that affected tectonism during the Ouachita and the Alleghenian orogenies of the eastern Gulf Coast and southeastern United States. The northernmost upthrown block is considered part of an ancient passive continental margin developed during the late Precambrian. The southern downthrown block is deformed by left-lateral transverse faults active during the Ouachita Orogeny. The Ouachita Orogeny may have deformed terrain farther east than the Black Worrior basin. These transverse fault blocks were buttressed by the footwall of the extensional fault system. These left-lateral faults extending from Florida and Georgia into Alabama, Mississippi, and southern Arkansas.

The Appalachian-Ouachita rifted margin of southeastern North America

Promontories and embayments along the late Precambrian-early Paleozoic Appalachian-Ouachita continental margin of south-eastern North America are framed by a northeast-striking rift system offset by northwest-striking transform faults. Inboard from the continental margin, basement fault systems have two sets of orientation; one is northeast parallel with rift segments, and the other is northwest parallel with transform faults. Late Precambrian clastic and volcanic syn-rift rocks overlie Precambrian basement rocks along the Appalachian Blue Ridge. Lower Cambrian sandstone at the base of a transgressive passive-margin succession over-steps the rift-fill successions and basement rocks, defining the time of transition from an active rift to a passive margin along the Blue Ridge. Locally thick Early Late Cambrian and older sedimentary rocks fill downthrown blocks of the intracratonic Mississippi Valley-Rough Creek-Rome graben system and Birmingham basement fault system. These basement fault systems, which indicate north-west-southeast extension like the Blue Ridge rift, are overstepped by Upper Cambrian strata. The northwest-striking Southern Oklahoma fault system is interpreted to be a transform fault that propagated into the continent from the Ouachita rift. Early and Middle Cambrian rift-related igneous rocks along the fault system and adjacent Precambrian basement are overstepped by Upper Cambrian sandstone. The differences in age of rift-related rocks suggest a spreading-center shift at the beginning of the Cambrian Period from the Blue Ridge rift to the Ouachita rift southwest of the Alabama-Oklahoma transform fault. From Early to Early Late Cambrian, a small component of extension propagates north-eastward to form the intracratonic fault systems northeast of the transform fault, but most of the extension of the Ouachita rift was transformed along the Alabama-Oklahoma transform fault to the Mid-Iapetus Ridge outboard from the Blue Ridge passive margin.

The Tartan Field, Block 15/16, UK North Sea

Abstract Tartan Field lies 118 miles northeast of Aberdeen in UK Block 15/16 on the southern flank of the Witch Ground Graben. The Field consists of two major southerly-dipping rotated fault blocks, the 'Upthrown Block' to the south and the 'Downthrown Block' to the north. The primary reservoir comprises Oxfordian-Kimmeridgian shallow marine sandstone of the Piper Formation deposited during a regional marine transgression. The secondary reservoir consists of Volgian turbidites (the 'Hot Lenses') within the Kimmeridge Clay Formation. The accumulations have been sourced from maturation of the Kimmeridge Clay Formation below approximately 10000 ft in adjacent basins. The Piper Formation exhibits markedly different petrophysical properties within each block. A relatively homogeneous intergranular porosity system is present throughout the oil zone of the 'Upthrown Block'. Porosities are lower and more variable in the 'Downthrown Block' as a result of cementation and the presence of an intense compaction fabric. The trapping mechanism is a combination of structural and stratigraphic elements. The Field was discovered in December 1974 by the 15/16-1 well which penetrated 263 ft of Piper Formation sandstone full to base with oil. A further ten straight holes and three sidetracked holes delineated the structure. Tartan Field came onstream in January 1981 and is currently producing 30000 BOPD through eight platform producers, assisted by six subsea water injection wells. Ultimate recoverable reserves are currently estimated at 116 MMBBL of crude oil.

Subsurface Models of Coal Occurrence, Oak Grove field, Black Warrior Basin, Alabama

Subsurface investigation of coal occurrence in the Black Creek-Cobb interval of Oak Grove field is based on cross sections and isopach maps made from more than 500 density logs. This study was designed to identify styles of coal occurrence in the Black Warrior basin to aid in coalbed-methane exploration and production. Coal occurrence in parts of the Black Warrior basin may be characterized in terms of end-member fluvial and structural control. Fluvial processes apparently were the major controls on coal occurrence in the Black Creek cycle, where sandstone and coal thickness are inversely related. Additionally, occurrence of thick sandstone sequences above the thickest coal beds suggests that peat compaction provided sites for channel avulsion. In the Mary Lee and Cobb cycles, more coal beds occur in a downthrown fault block than in an upthrown block, and in the Pratt and Cobb cycles, the thickest coal beds occur on the downthrown side of a fault. Only in the Mary Lee cycle, where thick peat accumulated in an abandoned tributary system, is coal thickest on the upthrown block. Most coal beds in Oak Grove field are thickest on the downthrown block because differential subsidence apparently promoted peat accumulation. Clastic influx favors bedsmore » splits in the downthrown block, but joining of beds in the Pratt cycle may reflect sheltering by the fault. In the Mary Lee cycle, in contrast, channel incision evidently provided local relief sufficient for thick peat to accumulate in lows on the upthrown block. Although fluvial and structural processes result in varied styles of coal occurrence, models of coal occurrence have resulted in a predictive framework that may aid in strategic well siting and completion.« less

On a Mechanism of Clay Smear Emplacement in Synsedimentary Normal Faults

Faults that contain clay smears that may act as effective barriers against fluid flow. It is therefore of interest to understand the mechanism by which clay smears are emplaced in fault zones. Such a mechanism was inferred by the authors from observations made in the openpit browncoal mines at Frechen near Cologne, Germany, where synsedimentary (growth) faults were exposed by openpit mining in a paralic sequence of gravel, slightly consolidated sands, shales, and lignite of the Late Tertiary Rhine delta. The fault zones at Frechen contain persistent clay intercalations, a few millimeters to as much as 1 m in thickness, derived from extremely plastic shale beds in the upthrown and downthrown blocks, and smeared-out-usually without substantial admixing of sandy material from the fault walls - over distances as great as 70 m in dip direction. In the Frechen mine, these 'clay smears' are found to seal faults against transverse groundwater flow. The effectiveness of this important fault sealing mechanism, which is likely to operate only in soft sediments and at sufficiently slow fault slip velocities, can be quantified in an approximative way in terms of a dimensionless ratio of the clay extrusion rate over fault slip velocity. While it appearsmore » from this study that clay smears start to form at shallow depth, field observations reported elsewhere suggest that they may grow and preserve their sealing properties during subsequent burial.« less

Mississippi Canyon 194, Gulf of Mexico: Development Case History Geological Summary

Mississippi Canyon 194 field (Cognac) was discovered in 1975 by Shell Offshore Inc. and its partners. Development was completed during 1981. Production to date totals in excess of 150 MMBE. During the last five years, production has been sufficient to impact geological interpretations of the field. Recently a 3-D geophysical survey was acquired, processed, and interpreted. This total data package of production, petrophysical, geological, and geophysical information has been used in planning a 20-well redevelopment program completed in 1990. The field is an anticlinal nose that plunges north from a major salt feature south of the field. The Cognac structure is situated in the downthrown block of a major south-dipping growth fault. The I, J, and K sands contain the majority of the hydrocarbons. These sands are part of a 1500-2000-ft thick section of Pliocene mini-basin deposits along the present-day shelf edge. Geological interpretations of the field include structural mapping of the pay horizons and gross and net sand isopachous maps. Mapping, in conjunction with core data, log shapes, and other geological indicators, has aided in determining environments of deposition. This paper will demonstrate how understanding the environment of deposition of reservoir sands has led to improved seismic interpretations. Subsequently, more » improved geological/geophysical interpretation led to drilling outstep wells where there were no direct hydrocarbon indicators. The vertical distribution of sands is also better understood as a result of this work. It is expected that this integrated approach to better understand the reservoir will lead to improved recovery. « less

Hinge-Line Growth Faults and the Expanded Frio Formation of the Coastal Bend Area of Texas

ABSTRACT Southern portions of the Coastal Bend Area of Texas from southeastern Calhoun County to southern Aransas County are well south and east of the fairway sand development of the Frio formation. Down-dip Frio formation sands in these counties are usually dirty, shaley thin-bedded sands; or there is no sand development whatsoever. Located ten to twelve miles inland from the coast, there are a series of large growth faults, striking generally parallel to the coast, which control and rejuvenate sand deposition on the downthrown blocks of these regional down to the coast faults, causing a significant increase and expansion in the amount of Frio formation deposition due to movement on the fault contemporaneously with deposition. Sand deposition was the result of longshore currents, moving clastics parallel to the coast, and depositing them in a littoral to neritic environment. On the downthrown block of these hinge line growth faults in areas of local, anticlinal development, the crest of the structures are devoid of sand. As the section is traversed in the subsurface northwest from the anticlinal crest into the trace of the growth fault plane, there is a substantial increase in the amount and quality of the sands deposited, most of which contain commercial hydrocarbon deposits when associated with local structure. These sands vary in physical characteristics from shaley, thin-bedded sands to moderately permeable, well developed sands varying in thickness from a few feet to several feet. Northwest Powderhorn Field in Calhoun County at the northeastern extremity of the area and the Nine Mile Point Field at the southwestern extremity of the area in Aransas County are two examples of this type of down dip Frio deposition and structures associated with producing gas-condensate fields.

Evidence for Unconformities in the Cenozoic of the Central and South Texas Continental Shelf from Regional Electric Log and Seismic Cross Sections: ABSTRACT

ABSTRACT Regional stratigraphic studies conducted by the Minerals Management Service, Gulf of Mexico, Outer Continental Shelf Region, on the offshore Texas shelf have identified at least eight major and several minor unconformities within the Cenozoic sediments. The identified erosional features associated with these unconformities include structural truncation, valley incision, and submarine canyon formation, which are the characteristics of a Type I unconformity as defined by Vail et al. (1977). There is also evidence for onlap by the overlying beds. The major unconformities are observed on the upthrown and downthrown sides of regional growth fault systems which are roughly parallel to the Texas coast. Where data quality permits, the major unconformities are identified over most of the Texas shelf. The minor unconformities are observed in the expanded sections on the downthrown blocks of major growth faults only. Although these erosional surfaces may also be Type I unconformities, no evidence of erosion is observed on the upthrown sides of the major growth faults. The unconformities of Oligocene and lower and middle Miocene age were initially interpreted from electric log correlations and subsequently verified on seismic data. The upper Miocene and younger unconformities were first recognized through seismic stratigraphy studies and then tied to electric log correlations. Most of the unconformities are observed near partially eroded diapiric ridges located along what was the outer shelf/upper slope area at the time of the erosion. A major upper Miocene unconformity is observed in the inner shelf/transitional environment of deposition in Matagorda and Mustang Island areas. This erosional surface was most likely formed by wave erosion during the subsequent transgression after deposition of the Discorbis 12 age sediments. There is also evidence of submarine scour by turbidites or massive slumping on this upper Miocene unconformity in the area of lower slope deposition. No evidence of erosion of outer shelf or upper slope sediments is apparent. Mapping in the lower Miocene trend delineated shale ridges plunging in a basinward direction perpendicular to regional strike. Electric log correlation of regional markers, which represent flooding surfaces, facilitated correlation of the unconformities on the seismic sections across these ridges. Occasionally very detailed seismic stratigraphy studies were necessary to establish which flooding surface should be used as the regional marker across large faults and ridges. End_of_Record - Last_Page 51--------

Mechanics of graben formation in crustal rocks: A finite element analysis

Small (1–10 km wide) grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites. In spite of their frequent occurrence, there is no fundamental understanding of how they form, other than that they are characteristic of extensional terranes. This paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults (a graben) is the energetically most favorable fault configuration in elastic‐brittle rocks subjected to pure extension. A corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional, or of nonhomogeneous characteristics of the rock. The analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two‐dimensional finite element model. It is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions, one near the surface of the downthrown block and the other near the fault tip. The region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault's depth. The proximity to failure actually increases here, and a secondary fault develops. An antithetic secondary fault would link this region with the other failure zone near the fault tip. To demonstrate the preference for antithetic secondary faults more rigorously, we show that this fault configuration results in a smaller total strain energy than the synthetic configuration. In addition, a two‐layer model is used to show that the graben's width is only weakly dependent upon the presence of mechanically distinct layers in the crust. Both weak‐over‐strong and strong‐over‐weak configurations are investigated. The depth of the initial fault and the thickness of the top layer are varied, and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface. This analysis indicates that the depth of the initial fault is the major factor controlling graben width.

Vertical movement in orogenic belts and the timing of metamorphism relative to deformation

Summary Any phase of deformation in an orogenic belt that causes relative vertical movements of bodies of rock by a few kilometres or more should have a recognizable imprint in the metamorphic history. The effect seen in a P-T-t path will depend on whether the rock body examined moved relatively up or down in the orogenic pile. An upthrown block is predicted to cool soon after the onset of movement, but the resulting P-T path will be insignificantly different to that resulting from a cycle of thickening and erosion. A downthrown block will undergo renewed heating after movement, and evidence of the earlier metamorphic history will probably be overprinted. It is suggested that the recognition of phases of relative vertical movement in an orogenic belt is best made by combined studies of deformation-metamorphism textural relationships, and P-T-t paths. The Lukmanier Pass area of the Alps illustrates the textural patterns predicted: here, a phase of deformation immediately post-dates the metamorphic peak, suggesting uplift of the area relative to the Helvetic nappes to the north.

Growth Faults in Oligocene Deltaic Sediments and Their Influence on Hydrocarbon Accumulation in Oil Fields of Upper Assam, Northeastern India: ABSTRACT

Accumulations of oil and gas have been found in the Oligocene sediments represented by the Barail Group in the Upper Assam, northeastern India. These deltaic sediments are composed of a thick sequence of an arenaceous member resting on bluish gray shale (the Eocene Kopili Formation) followed by an alternating sequence of sandstone and shale/mudstone/coal. Seismic and subsurface geologic evidence has shown the presence of a series of growth faults in a down-basin direction partitioning the basin into a series of subparallel fault blocks. The faults, which become listric with depth, trend northeast-southwest. Some of the faults possess a slanted S-shaped profile. These faults have steep zones (50/degrees/-70/degrees/) near the top and gradually shallow at depth, with sequences on the downthrown block overthickened. Structural closures on the downthrown side of the growth faults afford an ideal condition for hydrocarbon entrapment. Multiple reservoirs are present that may consist of one or more elliptical to crescent-shaped bar sands in the arenaceous sequence or isolated sand lenses in the upper Barail Group. The structural closure of the bar sands is generally greater than that of the sand lenses. The growth faults in the Upper Assam area may have been initiated by a combination of more » (1) sedimentary column of density inversion, i.e., the denser sands overlie less dense clays, (2) rapid prograding sedimentation in a deltaic environment, and (3) the effects of both incipient and pronounced plate motion that occurred during the Eocene through Oligocene. « less

Differential Fault-Related Early Miocene Sedimentation, Bayou Hebert Area, Southwestern Louisiana

Well logs from the Bayou Hebert area in southwestern Louisiana show that most of the differential sedimentation associated with one zone of contemporaneous faults cannot be attributed to simple expansion. Rather, the lower Miocene sequence on the downthrown side of the fault zone has grown principally by the episodic deposition of sedimentary units that have no correlative counterparts on the upthrown side. Such units range up to about 2,000 ft (600 m) thick near faults, and most are laterally discontinuous over distances of several kilometers. The isolated downthrown intervals lie in different parts of the sequence at various places along the strike of the fault zone, indicating lateral and temporal variation in subsidence rates in the downthrown block. The intervals may represent times of erosion or sediment bypass updip of faults. Isolation of thick intervals on the downthrown block adds potential for unexpected reservoir units and may enhance the development of combination traps associated with contemporaneous faulting and deposition characteristic of growth faults. Overlapping of the tapered edges of multiple laterally adjacent intervals may also create potential for small flexural structures in the areas of overlap.

The development and destruction of porosity within Upper Jurassic reservoir sandstones of the Piper and Tartan Fields, Outer Moray Firth, North Sea

AbstractUpper Jurassic Piper Formation sandstones of the Outer Moray Firth Basin, UK North Sea, form the main reservoir for the Piper and Tartan Fields, and are sealed by either the Kimmeridge Clay or Cretaceous marls. The main reservoir sandstones can be broadly considered as stacked coarsening-upward units that accumulated in a predominantly high-energy, shallow marine complex with depositional waters of inferred normal marine salinity, Eh and ionic composition. The amount of porosity and its distribution throughout these sandstones differs between the individual structural entities of the Piper and Tartan Fields. On the Piper structure all the sandstones are highly porous. Oil-zone porosities average 25% and are only slightly reduced in the water zone. Sandstones in the upthrown block of the Tartan structure are also generally porous but porosities are typically reduced to below 20%. Porosities are the most variable in the Tartan downthrown block and in the 15/17-9 well, off-structure Piper, where porous sandstones with between 14 and 18% porosity are associated with highly compacted and cemented sandstones with <5% porosity. Thin-section and SEM textural evidence indicate that much of the porosity in these sandstones is secondary, produced through the dissolution of an intergranular cement that provided support for the detrital framework, was peripherally replacive with respect to most detrital grains, and extensively replaced feldspars. Nodular concretions and sporadic, irregular crystals of ferroan calcite are inferred to be the remnants of this intergranular cement. The distribution of porosity zones in the Tartan downthrown block is related to the present structure and oil-water contact, supporting the interpretation of a secondary origin for much of the present porosity. Highly compacted sandstones interbedded with the porous sandstones result from a combination of early compaction, brittle framework collapse following the generation of secondary porosity and late grain-to-grain contact dissolution. Aggressive fluids responsible for the generation of the cement dissolution porosity are inferred to have been expelled from the Kimmeridge Clay in the adjacent Witch Ground Graben. The preservation of secondary porosity in Piper Formation sandstones is due to the relative timing of secondary porosity generation and subsequent hydrocarbon migration.

Models for Fine-Scale Movements Along Growth Faults Associated with Climate/Sea Level Changes: ABSTRACT

Along the northern Gulf of Mexico, the deglaciation phase of a Pliocene-Pleistocene glacial cycle produced up to a tenfold increase in waters available for sediment transportation to continental margins. With lowered sea levels during glacial maxima, sedimentary pulses are deposited along the shelf break and on the slope. High-energy deposition promotes foundation instability. Growth faults then originate along the shelf break. Increased lithostatic pressure due to rapid deposition on underlying unconsolidated and semiconsolidated sediments, semiplastic salt, and marine clays promote flow that accelerates compaction, fluid migration, growth fault movements, and salt tectonics. Sedimentary depositional cycles of deca- and centenary-millennia are recorded throughout the Phanerozoic. Planetary orbital motions, controlling the amount of incoming solar radiation and lasting 20,000, 40,00, and 100,000 years, apparently cause these depositional cycles via climate/sea level fluctuations. When sea level is low, sediments are deposited about or beyond the shelf break. Increased lithostatic pressure promotes movement on the fault, deposition on the downthrown block, accelerated compaction of sediments, and fluid expulsion and migration. When sea level is high, sediment deposition is along the upper and middle shelf. Fault activity should be minimal or nil. Thus, the geologic episodes that record the greatest amount of downthrown block growthmore » may also be punctuated by bursts of large drops in sea level. Onshore Louisiana production is usually greatest from maximum growth downthrown blocks.« less