ALBERT

Description: We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195°C to 268°C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10 •2 md for Y against 10 •3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Description: This work discusses concepts related to the occurrence of salt along weakness planes, such as faults and fractures, which resemble igneous intrusions and may result in peculiar seismic features. We suggest that mechanisms for the formation of these structures basically involve the creation of extensional faults (commonly associated with crestal collapse grabens), which are rotated and migrated to structural flanks by domation, creating interesting seismic features here referred to as halokinetic rotating faults. At the time of their formation, some of these faults may be incipiently intruded by salt as a way of relieving sporadic intense internal overpressure episodes in the salt body, by regional compression, and/or by buoyancy effects compensating the density difference between salt and surrounding sediments. The relatively low overburden pressure at the crest of the diapir and the original high dip angles of these fault planes favor salt intrusions near the diapir apex. The process may occur in several cycles along the salt dome evolution, creating several generations of salt apophyses positioned in the diapir apex and flanks, resulting in different dips and areas of extension. These intrusions sometimes resemble the branches of Christmas tree structures, which are commonly formed by extrusive mechanisms. Although well and seismic data point to the occurrence of salt along fault planes, we recognize that salt is not a low-viscosity fluid, and the mechanisms to allow its penetration along fault planes remain unknown. Some of the possible mechanisms, which are commonly associated with a later phase of regional compression, are discussed in this work. The implications for petroleum exploration may have been overlooked in the recent exploration campaigns in the deep-water regions of the Brazilian margin. Halokinetic rotating faults, when partially filled with salt, are sometimes responsible for common pitfalls observed in seismic and well data interpretation. When fault planes present subhorizontal dips and high reflectivity, caused by the presence of salt, they have been mistakenly interpreted as flatspots, a well-known seismic hydrocarbon indicator. When drilled and proved to correspond to thin evaporite intervals in well data, these salt apophyses have also been misinterpreted as younger localized evaporitic events overlying the main salt body.

Description: The Cretaceous rocks of Florida have been recognized as potentially suitable reservoirs for geologic carbon dioxide (CO 2 ) sequestration. Specifically, the upper member of the Upper Cretaceous Lawson Formation, together with the lower part of the Paleocene Cedar Keys Formation, is presented here as a potential composite CO 2 storage reservoir that is mainly composed of porous dolostone sealed by thick anhydrites of the overlying middle Cedar Keys Formation. Many of the porous intervals within the Cedar Keys-Lawson storage reservoir display lateral continuity and have an average porosity range of 20%–30%. The estimated CO 2 storage capacity for the reservoir is approximately 97 billion t of CO 2 , which means the Lawson and Cedar Keys Formations composite reservoir could potentially support CO 2 sequestration for hundreds of large-scale power plants in the southeastern United States for their entire 40-yr lifespan. Because most of the previous research on the Lawson Formation is concentrated in north-central and northeastern Florida and southern Georgia, this study further characterizes the formation and its CO 2 sequestration potential in south-central and southern Florida.

Description: Geochemical reactions that may occur on CO 2 injection into a sandstone formation in Missouri (MO) were investigated by means of geochemical modeling. Five possible injection sites were considered: two in the northwestern part of the state, two in the northeastearn part, and one in the southwestern part. The Geochemist Workbench software was used to investigate solubility trapping and mineral precipitation. Modeling was performed for two periods: an injection period of 10 yr and a postinjection period where the reactions proceeded to equilibrium. The work presented substantial challenges. Among them are uncertainty in kinetic constants for the dissolution and precipitation of minerals on CO 2 injection. Model results include equilibrium values for CO 2 stored via solubility trapping ranging from 49-g CO 2 /kg free formation water in Northeast MO to 78-g CO 2 /kg free formation water for Southwest MO. Mineral trapping is significantly lower, between 2.6- and 18.4-g CO 2 /kg free formation water. The model shows siderite and dawsonite as the major carbonate minerals formed, in this order. On a volumetric basis, northwest MO sequestration values were slightly greater than those obtained for northeast MO because of the somewhat greater depth and higher injection pressure at the injection target (Lamotte Sandstone) at the northwestern sites. However, the greater thickness of the aquifer for the northeastern sites provided overall greater sequestration capacity. Greene County was altogether unfit for sequestration because of the low total dissolved solids value of the formation water.

Description: We present a new hypothesis for the Jurassic plate-tectonic evolution of the Gulf of Mexico basin and discuss how this evolution influenced Jurassic salt tectonics. Four interpretations, some based on new data, constrain the hypothesis. First, the limit of normal oceanic crust coincides with a landward-dipping basement ramp near the seaward end of the salt basin, which has been mapped on seismic data. Second, the deep salt in the deep-water Gulf of Mexico can be separated into provinces on the basis of position with respect to this ramp. Third, paleodepths in the postsalt sequence indicate that salt filled the Gulf of Mexico salt basin to near sea level. Fourth, seismic data show that postsalt sediments in the central Louann and the Yucatan salt basins exhibit large magnitudes of Late Jurassic salt-detached extension not balanced by equivalent salt-detached shortening. In our hypothesis, Callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust. After salt deposition ended, rifting continued for another 7 to 12 m.y. before sea-floor spreading began. During this phase of postsalt crustal stretching, the salt and its overburden were extended by 100 to 250 km (62–155 mi), depending on location. Sea-floor spreading divided the northern Gulf of Mexico into two segments, separated by the northwest-trending Brazos transform. The eastern segment opened from east to west, leaving the Walker Ridge salient in the center of the basin as the final area to break apart. In some areas, salt flowed seaward onto new oceanic crust, first concordantly over the basement as a parautochthonous province, then climbing up over stratigraphically younger strata as an allochthonous province.

Description: Three aspects of basement structure and rift-related salt distribution have especially influenced the evolution of the deep-water northern Gulf of Mexico: (1) creation of a basement high (Toledo Bend flexure), separating a chain of interior basins from the central Louann salt basin, (2) segmentation of the central Louann salt basin by the Brazos transfer fault into eastern and central domains, and (3) salt provinces formed during basin opening. The Toledo Bend flexure was reactivated as a hinge during the Cenozoic uplift of the North American craton. This uplift triggered gravity gliding, forming fold belts in the seaward parts of the continental margin. The geometry of the Toledo Bend flexure influenced the position of these fold belts. The Brazos transfer fault separates the west sector of the study area from the central and east sectors. Most of the salt in the deep-water northern Gulf of Mexico lay in the central sector, which sourced most of the Sigsbee salt canopy. The western sector was narrower and was subdivided by the East Breaks basement high. Splitting the Callovian salt basin in two as the gulf opened created a southward-thinning wedge of salt at the seaward end of the northern Gulf of Mexico. We divide this wedge into a series of provinces on the basis of the geometry of the base of the deep salt. Original salt thickness influenced diapir location, the geometry of the Sigsbee canopy, the geometry and style of later compressional fold belts, and petroleum systems.

Description: Recent ultradeep exploration in the northern Gulf of Mexico has revealed a broad diffuse zone of salt-cored folding beneath the present continental shelf. This zone is a pillow fold belt, where salt pillows grew halokinetically and were then mildly shortened. Below the Louisiana shelf, a contractional early-to-late Miocene pillow fold belt is separated by a partly welded canopy from an overlying early Miocene–to–Pliocene extensional system. This anomalous juxtaposition raises two paradoxes: (1) Why was mid-Miocene shortening close to the Miocene shelf break, where extension is expected? and (2) Why did shortening below the canopy overlap in time with extension above the canopy? Coastal uplift can explain both paradoxes. Cenozoic uplift and exhumation of the north rim of the Gulf of Mexico created the observed coastal offlap and truncation around the rim. Uplift tilted the continental margin and overpowered the influence of the paleoshelf break, causing shortening much farther updip than before uplift. Physical models confirm that this hypothesis is mechanically sound. Our other models had two stacked detachments, each pinned in different locations. Because of this, deep shortening below the canopy was coeval with shallow extension above the canopy. The deep detachment was pinned far inland, equivalent to the uplifted continental interior. Extension above this deep detachment was partly balanced by shortening far downdip to form a pillow fold belt where a network of thrusts linked the squeezed pillows. In contrast, the shallow extensional system above the canopy was pinned farther seaward, equivalent to the upper continental slope.

Description: The process and mechanisms of secondary hydrocarbon migration in the Tazhong uplift, Tarim Basin, were investigated based on the analysis of the regional structure and by integrating geologic, hydrodynamic, and geochemical parameters. Parameters successfully analyzed included the fluid potential, fluid properties, production outputs, and diamantane index. The results indicated that hydrocarbons migrated into the Tazhong uplift from the northern part of the Manjiaer depression through a series of injection points (IPs) during four orogenies, that is, the early Caledonian (510 Ma), the late Caledonian (439 Ma), the late Hercynian–Indosinian (290 Ma), and the Yanshanian–Himalayan (208 Ma). A total of six IPs were identified at the intersections of the northeast-trending faults and the northwest-trending flower strike faults. The hydrocarbons migrated from the IPs into traps along regional trends from northwest to southeast and from northeast to southwest. The hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the Tazhong uplift. With increasing distance from the IPs, daily hydrocarbon production decreases, and the hydrocarbons become progressively heavier and display lower gas:oil ratios.

Description: Determination of turbidite event magnitude and frequency remains subjective and difficult to define. This is because turbidite sedimentation events commonly include both sand and mud, with the mud component commonly excluded from bed thickness studies because of the inability to establish a genetic link to the turbidity current. Pelagic mudrock is defined as fine-grained marine sediment derived primarily from biogenic particles, whereas hemipelagic mudrock includes both biogenic and terrigenous particles. Unfortunately, these compositional definitions do not account for differences in depositional process. Scanning electron microscopy, field emission scanning electron microscopy, and x-ray diffraction analyses of 70 samples from El Rosario Formation outcrops (Baja California, Mexico) and core from the Woodford Shale (Oklahoma) illustrate this distinction. Furthermore, these laboratory measurements are calibrated to 192 outcrop samples to provide a robust method for field identification of clay fabric and mineralogy to define turbidite sedimentation units. Pelagites show organized layering of clay platelets, few flocculates, and a lower proportion of high-density minerals. Hemipelagites have disorganized and chaotic clay fabrics characterized by visible flocculates and contain a higher proportion of denser particles. There may also be a corresponding change in clay mineralogy, for example, smectite in pelagites versus kaolinite in hemipelagites. These results indicate a settling velocity greater than shear velocity in pelagites, whereas hemipelagites record the opposite condition. Turbidity currents support and suspend denser grains, generate disorganized and chaotic clay fabrics, and provide more time for flocculation. Discrimination between pelagites and hemipelagites has important implications in the determination of turbidite event frequency and magnitude, which affects vertical connectivity and continuity of sand, deposited from multipartite turbidity currents. In addition, distinction between pelagites and hemipelagites provides a better understanding of mudrock reservoir architecture.

Description: The Molasse deposits of the Central Eastern Alps are partly incorporated into a fold and thrust belt that recently has come into exploration focus. The structural style and timing of deformation varies significantly alongstrike. Regional three-dimensional seismic and well data interpretation indicate three different structural segments from east to west: (1) The Sierning imbricates have a decollement close to the base of the Molasse sequence and consist of varying numbers of thrust sheets alongstrike. Early Miocene shortening of the Molasse is at least 6.2 km (3.9 mi). Overthrusting of the internal Penninic and Helvetic wedge since the Oligocene accommodated at least 25 km (15.5 mi) of additional shortening. (2) The Regau segment is dominated by one to two small thrust sheets above a shallow detachment. This segment is dominated by overthrusting of the Alpine wedge. (3) The Perwang imbricates consist of an Oligocene wedge with complex deformed thrust sheets above a detachment horizon in Upper Cretaceous marls. Minimum shortening in the imbricates is 18.5 km (11.5 mi) with overthrusting 33.3 km (20.7 mi). All shortening estimates have an uncertainty of approximately 20% to 35%. The laterally varying thrust-belt architecture results from predeformational conditions (e.g., sediment thickness, mechanical stratigraphy, and basement dip). In the Sierning imbricates, hydrocarbon trap definition and charge issues are exploration risks. In the Regau segment, exploration is focused on the subthrust play. The Perwang imbricates have hydrocarbon shows but no economic discoveries. Charge and seal issues are the main risks. The petroleum systems in the context of the structural evolution are not yet fully understood.