ALBERT

Description: Submarine landslides are mass movements that transport sediment across the continental shelf to the deep ocean. This phenomenon happens when the shear stress exceeds the frictional resistance of the slope. We analyze a variety of seismic attributes to interpret large submarine slide blocks on the North Slope, Alaska. Results show that the slide blocks appear as mounds with scarps associated with them on the seismic section. The slide blocks vary in size, depending on their distance away from the shelf. The pattern of the slide blocks affects the overlying sedimentation. Geological feature: Submarine slide blocks Seismic appearance: Mound-like steep ramp and scarp characteristics on seismic sections; blocky and irregular features with sharp boundaries on the horizon slices and seismic attributes Features with similar appearance: Mass-transport deposits; Remnant blocks; Reef deposits; Submarine channels; Gullies Formation: Torok Formation Age: Cretaceous Location: North Slope, Alaska Seismic data: Obtained from the Alaska Department of Natural Resources, Division of Oil and Gas, through the tax-credit program ( State of Alaska, 2017 , http://dggs.alaska.gov/gmc/seismic-well-data.php ) Analysis tools: Seismic attributes (such as coherent energy, Sobel-filter similarity, dip magnitude, and dip azimuth) and geobody extraction

Description: Linear features at an acute angle with the flight direction are imaged as a series of aligned circular anomalies in the images of the Area 15 aeromagnetic survey, which covered part of the Brazilian southeastern region. These features are interpolation artifacts, a recurring problem found in airborne magnetic images that cause problems for qualitative and quantitative geophysical-geological interpretation. This imaging problem is attributed to spatial aliasing. By running simulations of magnetic data on a synthetic model, we physically proved that the interpolation artifacts from Area 15 are due to inappropriate survey design. Besides the most common expression of artifacts, we described a geologically non-coherent linear pattern as a new type of artifact. Supported by spectral analyses we shown that Area 15 aliased spectrum is similar to geological high-frequency magnetic features, which constitutes a motive for unearthing the correct geophysical signal. Thus, we made use of four techniques for removing the artifacts. The trend enforcement method was able to partially improve the images, while the inverse interpolation method was ineffective, apparently because Area 15 data are severely aliased. Both constrained coherence diffusion and multi-trend gridding methods were able to significantly reduce the presence of artifacts. Despite the high-frequency attenuation, these tools adequately enhanced the magnetic trends and minimized the artifacts. Therefore, the improved images are better suited for a reliable geological interpretation.

Description: Seismic survey data collected for coal gas exploration show that there are many collapse columns distributed in the subsurface of Qinshui Basin, China. The interesting features of the collapse columns are observed by the seismic attributes, including the circular discontinuous patches on the horizon of the Shanxi Formation and multiple parallel discontinuities in vertical profiles of amplitudes. We speculate that the wide presence of these collapse columns are point constraints for the migration and accumulation of coal gas on a large scale. Geological feature: Collapse columns within coal reservoirs Seismic appearance: The coherence illuminates circular/oval discontinuities on the horizon of the Shanxi Formation; the vertical amplitude profiles show cylindrical/funnel-shaped discontinuities. Alternative interpretations: Fault damage zones; velocity pulldown from the overburden Features with similar appearance: Fault-karst in carbonate reservoir; reef pinnacles Formation: Permian Shanxi Formation and Carboniferous Taiyuan Formation Age: Late Permian Location: Qinshui Basin in Shanxi, north-central China Seismic data: Provided by PetroChina Huabei Oilfield Company Contributors: Zonghu Liao, Lin Zhang, and Lianbo Zeng Analysis tools: The seismic amplitude and attribute of coherence from the seismic survey (prestack time migrated)

Description: To quantitatively image fractures with high resolution, we develop an elastic least-squares migration (LSM) algorithm coupled with linear-slip theory, which accurately addresses seismic wave interaction with thin structures. We derive a linearized waveform inversion using the Born approximation to the boundary integral equation for scattered waves, including linear-slip interfaces for P-SV and SH wavefields. Numerical modeling tests assuming a laboratory-scale fracture where a 20 cm long fracture is illuminated by waves with 50 KHz center frequency, show that the proposed LSM successfully estimates the fracture compliances. Furthermore, due to the presence of coupling compliances at the fracture, the results using the proposed LSM show better images than those using the conventional LSM estimating Lamé constants. We also numerically illustrate that the proposed LSM can be successfully applied to dipole acoustic borehole logging data with 3 KHz center frequency for single-well reflection imaging of a 10 m long, dipping fracture embedded in a random background. Finally, we apply the LSM to laboratory experimental data, measuring PP reflections from a fluid-filled fracture. We confirm that the estimated fracture compliances correspond well to those estimated by earlier AVO inversion. Furthermore, the LSM resolves the spatially varying fracture compliances due to local filling of water in the fracture. Because the linear-slip theory can be applied to thin structures in a wide range of scales, high-resolution imaging results and estimated fracture compliance distributions will be crucial to further address small-scale properties at fractures, joints and geological faults.

Description: The Chang 7 oil layer from the upper Triassic Yanchang Formation is an important layer for hydrocarbon exploration. Most studies on the Chang 7 oil layer have focused on the source rocks, while research on the sandstone is still inadequate, especially on the petrography and geochemical characteristics. Using seven sandstone samples of the Chang 7 oil layer in the Yanhe profile, the grain-size analysis, major elements, trace elements, and rare earth elements were tested. The results find that the sandstone of fine-grained sediments of the Chang 7 oil layer is dominated by arkose with a minor number of lithic arkose. The range of grain size (Mz) is from 2.72 to 3.92 Φ, and the C value and M value of the sandstone samples suggest characteristics of turbidity deposition. The Al/Si ratios of all of the samples imply high clay mineral content. The results of trace and rare earth elements demonstrate the reducing condition, freshwater, and cold and dry weather. The provenance of the sandstone samples is mainly from island arc acidic volcanic rock, and the type of provenance is mixed with sedimentary rock, granite, and alkaline basalt. The tectonic background is continental island arc. This study provides a systematic geologic foundation for the formation of sandstone of Chang 7 oil layer in Ordos Basin.

Description: We have synthesized macroscopic data (cores) and microscopic data (thin sections and computed tomography images) to characterize volcanic oil and gas reservoir spaces in the Chagan Sag, Yin-E Basin, China. We also have assessed the controlling factors affecting the formation of these reservoirs. Primary pores significantly improve the reservoir quality because they enable the formation of secondary pores (dissolution pores) by allowing migration, alteration, and filling by inorganic and organic fluids. Consequently, the development of secondary pores modifies the permeability of the volcanic rocks. The formation of volcanic reservoirs is influenced by lithology, diagenic minerals, faults, and fractures. The size and abundance of pores, especially gas pores, varies with the lithology. Gas pores are consistently larger and more abundant in basalts than in andesites and tuffs. Moreover, the carbonates and chlorites readily dissolve in the vesicles and fractures, thus resulting in the formation of secondary pores. However, quartzes are not susceptible to dissolution in this study area. Further, faults and fractures are necessary for the formation of reservoir spaces and for oil and gas migration. The tomographic images provide 3D distributions of fractures, vesicles, and matrix dissolution pores. The results suggest that fractures connecting isolated gas pores provide favorable spaces for oil accumulation.

Description: Interpretation of recent, high-quality seismic data in the Gulf of Mexico (GOM) has led to competing hypotheses regarding the basin’s rift to drift transition. Some studies suggest a fault-controlled mechanism that ultimately results in mantle exhumation prior to seafloor spreading. Others suggest voluminous magmatic intrusion accommodates the terminal extension phase and results in the extrusion of volcanic seaward dipping reflectors (SDRs). Whereas it has been generally accepted that the plate motions between the rift and drift phases of the GOM are nearly perpendicular to each other, it has not been greatly discussed if the breakup mechanism plays a role in accommodating the transition in plate motion. We have developed a plate kinematic and crustal architecture hypothesis to address the transition from rift to drift in the GOM. We support the proposition of a fault-controlled breakup mechanism, in which slip on a detachment between the crust and mantle may have exhumed the mantle. However, we stress that this mechanism is not exclusive of synrift magmatism, though it does imply that SDRs observed in the GOM are not in this case indicative of a volcanic massif separating attenuated continental and normal oceanic crust. We support our hypothesis through a geometrically realistic 2D potential field model, which includes a magnetic seafloor spreading model constrained by recent published seismic data and analog rock properties. The 2D model suggests that magnetic anomalies near the continent-ocean transition may be related to removal of the lower continental crust during a phase of hyperextension prior to breakup, ending in mantle exhumation. The kinematics of breakup, derived from recent satellite gravity data and constrained by our spreading model and the global plate circuit, suggests that this phase of hyperextension accommodated the change in plate motion direction and a diachronous breakup across the GOM.

Description: The Frade field, located within the Campos Basin in the southeastern Brazilian margin, is a key oil field that produces from Oligo-Miocene turbidite reservoirs that derived their structural positioning due to the presence of an underlying salt diapir. The evolution of the Frade salt structure was examined using well data, selected 2D lines, and a 3D volume that were interpreted in detail focusing on the Aptian evaporite interval and its influence on the overburden. Analysis of the salt-sediment interaction indicated a complex deformation history that included five main stages of deformation, some assisted by tectonic reactivation episodes. (1) Post-Albian reactivation of a nearby north–northwest-south–southeast basement fault caused the Albian carbonate interval to fault, forming a west–northwest-east–southeast shear zone with a dextral strike-slip component. This movement initiated thin-skinned tectonics that offset the Albian carbonates and formed a pull-apart basin that accommodated a thick Late Cretaceous interval, which weakened the overburden and allowed for the initial formation of the Frade salt diapir. (2) Renewed diapir growth thickened and redistributed the Cenomanian-Maastrichtian sedimentary package proximal to the Frade salt anticline. (3) An initial and localized collapse of the Frade salt anticline occurred during early Paleogene extension. (4) Paleogene shortening caused the salt to flow, resulting in salt withdrawal in the southeast and diapir rejuvenation near its present-day apex, forming several inversion structures. In addition, the Paleogene shortening resulted in a low-relief anticlinal structure that rotated the turbidites into geometries favoring hydrocarbon accumulation. (5) A return to an extensional regime occurred during the late Oligocene/early Miocene. The results of this study provide a new insight into the development of strike-slip salt tectonic structures and show for the first time within the Campos Basin an Albian-level pull-apart basin that formed in association with salt tectonics.

Description: Faulting processes have created large damage zones with complex structures in the field; however, estimating the width and geometry of such fault structures in the subsurface is challenging due to a lack of data. Seismic attributes (e.g., coherence and variance) from seismic surveys have been used for the characterization of faults, but most cases do not detail the effectiveness of this approach. By using forward modeling and the associated seismic attributes of variance, four fault models of idealized damage zones are characterized and the frequency effect is evaluated on the width estimation of fault damage zones in the subsurface. The main results indicate that (1) the general geometric pattern of damage zones could be identified by using simulated amplitude and seismic variance with main frequencies of 10, 25, and 40 Hz; (2) the estimated widths of damage zones at a low frequency of 10 Hz are larger (up to twofold) than those at frequencies of 25 and 40 Hz; for large damage zones (〉400 m), the width is best estimated by a frequency of 25 Hz; and (3) scattering noise and diffraction around the fault are found in data at a high frequency of 40 Hz, which results in width overestimation of the damage zones by approximately 17%. The internal structures are difficult to distinguish as scattering noise and chaotic reflections dominate seismic signals. More factors that may influence the accuracy of damage zone width estimation via seismic attributes, include the bedding thickness, fracture density, and velocity. An in-depth understanding of this approach is useful in the application of seismic variance to characterize fault damage zones that may significantly control the fluid migration in the subsurface.

Description: The petrophysical properties of rocks have an important influence on shale quality. To characterize the difference of petrophysical properties between organic-rich and organic-poor shale reservoirs, we used the low-field nuclear magnetic resonance (NMR) technique and field emission scanning electron microscopy analysis after argon-ion polishing or natural section to measure porosity and permeability from six core samples from well SY6 in the Sangzhi block, Northwest Hunan province. Some information about pore types, pore structure, residual porosity, movable porosity, and permeability based on the T2 spectrums’ difference of organic-rich and organic-poor shale samples were discussed. The shale sample test results show that the main pores size is mesopore, which provide most of shale gas reservoir space. The continuous peaks demonstrated the pores’ connectivity better than the isolated peaks, and shale gas can migrate freely between these connected pores and fractures. The permeability of all samples calculated by the classic Coates model is extremely low, which is not conducive to the migration of shale gas. We evaluated the dominating factors of NMR porosity and permeability and found that the relationships between NMR porosity and permeability and total organic carbon content, quartz minerals, and clay minerals are not clear, which may be a comprehensive influence. The research results have important guiding significance for shale reservoir quality evaluation in this area.