The Tibetan Plateau is considered to be one of the best natural laboratories for seismological research. This study sought to determine the spatial variations of b-values in the western part of the Tibetan Plateau, along with its surrounding areas, and the relation with the region’s fault blocks. The study region lies within 27–36.5° N, 78–89° E, and its fracture structure consists of strike-slip faults, as well as normal and thrust faults. A catalog record from 2009–2019 provided 4431 well-centered earthquakes that varied in magnitude from 0.1 to 8.2 M. The record was obtained from China’s seismological network, which is capable of recording low magnitudes to analyze b-values in the study area. The key findings of this study are as follows: (1) the range of earthquake depth in the region was 0–256 km, with the depth histogram showing a high frequency occurrence of shallow earthquakes in the area; (2) a time histogram showed that the major earthquakes occurred between 2014–2015, including the notable 2015 Gorkha earthquake (M = 8.2); (3) the b-value computed in the study area was 0.5 to 1.6, but in most of the study area, the b-value ranged from 0.6 to 0.9, which was a low to intermediate value, due to the presence of strike-slip faults in the central part of the study area and underthrusting in the region (south of the study area); and (4) a high b-value was found in the northwestern and eastern regions of the area, which proved that the area is prone to small earthquakes in the near future. The study also showed that the central and southern areas of the study region had low to intermediate b-values, meaning that it is prone to destructive and massive earthquakes with high magnitudes, such as the Gorkha earthquake (southern part of the study area). Low b-values revealed the degree of variation in rock properties, including large stress and strain, a fractured medium, a high deformation rate, and large faults. Small b-values were observed when the stress level was high in the investigated region, which might be used to predict a massive high-magnitude earthquake in the near future.
The Sembar Shale formation in Lower Indus Basin Pakistan is thought to contain significant potential of unconventional resources; however, no detailed study has yet been carried out to quantify its potential. In conventional oil and gas exploration, reservoir rocks have been the main focus therefore, limited number of wells target the Sembar Formation. To explore its regional view, the seismic characterization of these shale is required. Generally, a poor correlation is generally observed between P-wave impedance and the reservoir and geomechanical properties of rocks, making it challenging to characterize them using seismic data. We present a workflow for characterizing the seismic derived unconventional prospect of the Sembar Shale using prestack seismic data along with well logs. The logging results of the two wells show that organic matter richness of well A is in high to very high values while, well B is in low to very low values. Considering the mineral composition and brittleness index evaluation the Sembar Shale in well A is brittle to less brittle in nature. The organic content, porosity, and brittleness index results in well A makes the Lower Cretaceous Sembar Formation favorable to be considered as a potential organic shale reservoir. Four sensitive attributes, derived through integration of the rock petrophysical, geochemical and geomechanical parameters, are correlated with P-wave impedance. The correlation of each sensitive attribute has been applied to characterize the Sembar Shale potential. These attributes are first-order indicators to depict organic matter, porosity and geomechanical properties. This attribute approach is further validated through rock physics modeling. The workflow presented in this study can be employed to assess unconventional reservoir potential of the Sembar Formation in other parts of the basin.
The successful production of unconventional resources such as shale gas is highly dependent on its two reservoir properties, organic matter and rock brittleness. High resolution spatially characterization of these two unconventional reservoir properties needs surface reflection seismic data. However, to delineate these two parameters on seismic scale is a challenging task because poor correlation is observed between these parameters and elastic properties of the rock. To encounter this adversity in current study we proposed effective attributes method in which organic shale reservoir properties are divided into their hard and soft elastic response. From the analysis of worldwide laboratory dataset, we find that hard and soft components have shown us much better linear correlation with P-and S-wave impedance. The proposed effective attributes, helped us to reduce the gap between unconventional reservoir properties and seismic characteristics. These attributes are the main controlling factor for rock elastic properties and exhibit information about hydrocarbon generation capacity and rock brittleness. A well data example from Sembar shale has also shown successful results for proposed effective attributes methodology. These attributes application on inverted P-wave impedance seismic data of employed organic shale reservoir have shown productive results to quantify its unconventional prospect on seismic scale. The approach used in this study can be confidently employed to assess unconventional reservoir potential in other parts of the world.
The sub-surface structural analysis to understand the geology and tectonics of an area is always useful to locate the hydrocarbon resources. Oil and gas based energy supplies have become a vital source for Pakistan, which is passing through an era of severe energy crisis. The study area, Buzdar block, in the southern Indus Basin is tectonically an extensional regime and is expected to have a huge hydrocarbon potential. In this study, we did the interpretation of the migrated seismic lines of the 872-SGR-527, 872-SGR-529, 872-SGR-531, 872-SGR-532 of Buzdar block, District TandoAllahyar, Sindh. The lines 872-SGR-529, 872-SGR-531, 872-SGR-532 were oriented W–E whereas the line 872-SGR-527 was oriented NW–SE. The obtained data was analysed and three reflectors were marked named top Khadro Formation, top lower Goru formation and top Chiltan limestone (probable). Through this study faults have been also marked on seismic lines which are normal faults by nature; collectively form horsts and grabens which is the evidence of effect of extensional tectonics in the area. Time contour maps were also generated. After that, time was converted into depth with the help of well velocity from VSP data for lower Goru formation and average velocity for Chiltan limestone (probable) from regression analysis. Finally, depth contour maps were generated which helped to know the basic mechanism of tectonic movement in the area. On the basis of present analysis we propose that a well may be drilled at Lower Goru formation near fault F1 on western side at a depth of 1370 meters and at 1290 meters near fault F4 on eastern side.
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