Mohammed Y. Ali

Prediction of magnitude of the largest potentially induced seismic event

We propose a method for determining the possible magnitude of a potentially largest induced seismic event derived from the Gutenberg–Richter law and an estimate of total released seismic moment. We emphasize that the presented relationship is valid for induced (not triggered) seismicity, as the total seismic moment of triggered seismicity is not bound by the injection. The ratio of the moment released by the largest event and weaker events is determined by the constants a and b of the Gutenberg–Richter law. We show that for a total released seismic moment, it is possible to estimate number of events greater than a given magnitude. We determine the formula for the moment magnitude of a probable largest seismic event with one occurrence within the recurrence interval (given by one volumetric change caused by mining or injecting). Finally, we compare theoretical and measured values of the moment magnitudes of the largest induced seismic events for selected geothermal and hydraulic fracturing projects.

Seismic stratigraphy and subsidence history of the United Arab Emirates (UAE) rifted margin and overlying foreland basins

Regional seismic reflection profiles and deep exploratory well data have been used to determine the structure and evolution of the United Arab Emirates (UAE) rifted margin and overlying foreland basins. We recognized three major tectonostratigraphic sequences in the seismic profiles. A lower rifted margin sequence which is characterised by an early syn-rift sequence of mainly Late Permian age that is overlain by a post-rift sequence of Triassic-Jurassic to Late Cretaceous age, and an upper foreland basin sequences of Aruma (Late Cretaceous) and Pabdeh (Palaeocene—Oligocene) Groups. Backstripping of the well data provides new constraints on the age of rifting, the amount of crustal and mantle extension, and the development of the UAE rifted margin and foreland basins. The tectonic subsidence and uplift history at the wells can be explained by a uniform extension model with at least two episodes of rifting punctuated by periods of relative tectonic quiescence and thermal subsidence. An initial age of rifting of 260 Ma and a stretching factor, β, of 1.2 was followed by second period of stretching with an initial age of rifting of 180 Ma, and β, of 1.1. This model accounts for the general exponential decrease observed in the backstripped tectonic subsidence. The model, fails, however, to completely explain the slow subsidence and uplift history of the margin during Late Triassic to Early Jurassic. We attribute this slow subsidence to combine effect of a sea-level fall and regional uplift which caused major unconformity particularly offshore western Abu Dhabi. The backstrip curves suggest that the transition from an extensional rifted margin to a compressional foreland basin occurred at ~90 Ma, which is within the range for the emplacement of the Semail Ophiolite (95–68 Ma). The history during this time is characterised by uplift followed by rapid subsidence. We attribute these differential vertical movements to orogenic loading and flexure of the Arabian rifted margin by the Semail Ophiolite and development of Aruma foreland basin.

Structure of the northern Oman Mountains from the Semail Ophiolite to the Foreland Basin

Abstract The northern Oman Mountains record the Late Cretaceous emplacement of the Semail Ophiolite and associated subduction trench and deeper-water sediments into a foredeep on the downwarped Arabian continental margin. In-sequence piggy-back thrusting was modified by later thrust sheets of Permian–Mesozoic continental slope sediments that breached the overlying allochthonous stack. Sections through these culminations show asymmetries linked to deep-seated faults perpendicular to the margin edge which created offsets (promontories) that influenced detachment. Seismic profiles across the foredeep indicate an unconformity at the top of the Mesozoic carbonate platform, the product of early flexural uplift, onlapped by sediments. A second onlap phase is linked with rapid subsidence during final emplacement of the allochthon. Passive margin conditions resumed during Maastrichtian time, conformable along the axis of the foredeep. A Paleocene–Eocene (Pabdeh) foredeep was initiated in the northern mountains, recorded on seismic profiles with another onlapping sequence. Subsidence and infill persisted into the Oligocene when compression, linked with the Zagros collision, resulted in uplift and en echelon folding, probably due to the reactivation of deep-seated faults. Post-Miocene erosion of one culmination exposed the cap of a gypsum/anhydrite intrusion. The origin remains uncertain, but derivation from the Neoproterozoic to Cambrian Ara Group salt is preferred.

Mapping basement structures in the northwestern offshore of Abu Dhabi from high-resolution aeromagnetic data

This paper presents a case study of mapping basement structures in the northwestern offshore of Abu Dhabi using high-resolution aeromagnetic data. Lineament analysis was carried out on the derivatives of the reduced-to-the-pole magnetic data, along with supporting information from published geologic data. The lineament analysis suggests three well-defined basement trends in the north–south, northeast–southwest, and northwest–southeast directions. The reduced-to-the-pole magnetic data reveal high positive magnetic anomalies hypothesized to be related to intra-basement bodies in the deep seated Arabian Shield. Depth to basement was estimated using spectral analysis and Source Parameter Imaging techniques. The spectral analysis suggests that the intruded basement blocks are at the same average depth level (around 8.5 km). The estimated Source Parameter Imaging depths from gridded reduced-to-the-pole data are ranged between 4 km and 12 km with a large depth variation within small distances. These estimated depths prevent a reliable interpretation of the nature of the basement relief. However, low-pass filtering of the horizontal local wavenumber data across two profiles shows that the basement terrain is characterized by a basin-like structure trending in the northeast–southwest direction with a maximum depth of 10 km. Two-dimensional forward magnetic modelling across the two profiles suggests that the high positive magnetic anomalies over the basin could be produced by intrusion of mafic igneous rocks with high susceptibility values (0.008 to 0.016 SI.

Attenuation modes from Vertical Seismic Profiling and sonic waveform in a carbonate reservoir, Abu Dhabi, United Arab Emirates

In this study, we derived accurate and high-resolution attenuation profiles using spectral ratio, centroid frequency shift, and seismic interferometry methods. We utilized high-quality vertical seismic profiling and sonic waveform data acquired in a carbonate reservoir located in Abu Dhabi, United Arab Emirates. The scattering profile of vertical-seismic-profiling data contributes significantly to wave attenuation that can be explained by high heterogeneity of the carbonate rocks. The scattering profile also correlates well with the reservoir lithology and fractured zones imaged by the Formation MicroImager. A tar mat zone occurs within the lower part of Arab D reservoir. This zone corresponds with a decrease in scattering attenuation. The tar mat may have filled the pores and made this zone less heterogeneous. Therefore, a decrease in scattering attenuation can be considered a potential parameter for tar mat detection. After removing the scattering effect, nonphysical negative intrinsic attenuation values still exist at certain depths. The most probable explanation for this is the three-dimensional scattering effect, which is not taken into account in this paper, and short-period upgoing waves. Seismic interferometry is less sensitive to the remaining scattered upgoing wave, which is why seismic interferometry method shows fewer negative values than the spectral ratio and centroid frequency shift methods. Compared with vertical-seismic-profiling attenuation, scattering attenuation estimated from sonic waveforms recorded in the reservoir zones is insignificant, and the intrinsic attenuation is almost equivalent to the total attenuation. We attribute this underestimation of the scattering attenuation to the sparse spatial sampling of the sonic logging data at 0.1524 m, which is not sufficient to appropriately estimate the scattering effect in heterogeneous media. The cross-plots between sonic attenuation and various petrophysical properties show slight dependence between the sonic attenuation and neutron porosity and resistivity in the reservoir zones. However, we can highlight from these plots two zones belonging to the Arab reservoirs. The lower zone corresponds to Arab D reservoir and displays higher sonic intrinsic attenuation than the upper zone (Arab A–C reservoirs) due to higher oil saturation. This highlights the sensitivity of the intrinsic attenuation to the oil saturation.

Mapping Basement Structures in the Northwestern Offshore of Abu Dhabi from High-resolution Aeromagnetic Data

This paper presents a case study of mapping basement structures in the northwestern offshore of Abu Dhabi using high-resolution aeromagnetic data. Lineaments analysis was carried out on the derivatives of the reduced to the pole (RTP) magnetic data, along with supporting information from published geologic data. The lineament analysis suggests three well-defined basement lineaments in the study area, which are mainly trending in the N-S, NE-SW and NW-SE directions. The RTP magnetic data reveals two high magnetic anomalies related to structurally-controlled highs in the deep seated Arabian Shield. Depth to basement was estimated using spectral analysis and suggests the two basement blocks are in the same average depth level (around 8.5 km).