ALBERT

Description: The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of the Bering Sea and consists of zones of complexly deformed accreted terranes. Along the northern portion of the peninsula, progressing from then orthwestem Bering Sea inland the Olyutorskiy, Ukelayat, and Koryak superterranes area acreted to the Okhotsk-Chukotsk volcanic-plutonic bell in northern-most Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlap terranes and units of the Koryak superterrane and constrains their accretion time with this region of the North America plate. Ophiolite complexes, widespread within the Koryak superterrane, are associated with serpentinite melanges and some of the ophiolite terranes include large portions of weakly serpentinized hyperbasites, layered gabbro, sheeted dikes, and pillow basalts outcropping as internally coherent blocks within a sheared melange matrix. Interpretation of magnetic anomalies allow the correlation of the Ukelayat with the West Kamchatka and Sredinny Range superterranes. The Olyutorskiy composite terrane may be correlated with the central and southern Kamchatka Peninsula Litke, Eastern Ranges and Vetlov composite terranes. The most "out-board" of the central and southern Kamchatka Peninsula terranes is the Kronotsky composite terrane, weil exposed along the Kamchatka, Kronotsky and Shipunsky Capes. Using regional geological constraints, paleomagnetism, and plate kinematic models for the Pacific basin a regional model can be proposed in which accretion of the Koryak composite terrane to the North America plate occurs during the Campanian-Maastrichtian, followed by the accretion of the Olyutorskiy composite terrane in the Middle Eocene, and the Late Oligocene-Early Miocene collision of the Kronotsky composite terrane. A revised age estimate of a key overlapping sedirnentary sequence of the Koryak superterrane, calibrated with new Ar40/Ar39 data, supports its Late Cretaceous accretion age.

Description: Quantifying transitions in seismic activity related to wastewater injection is an important step for accurate seismic-hazard assessment. It is also a challenging task due to the uncertainty in the relationship between injections and transitions; however, a consistent statistical analysis of instrumental seismic records allows for detecting and quantifying induced seismicity. In this study, we develop a statistical method for modeling a seismic sequence involving non-stationary-induced seismicity. It is composed of two steps: first, we select a model for the integrated seismicity (i.e., natural and induced) within the framework of the epidemic-type aftershock sequence via the Bayesian Model Comparison section. Second, we perform Bayesian inference within that model to assess the seismic activity and associated parameters. The method is applied to the analysis of the events from Oklahoma, demonstrating that it is able to provide a consistent representation of the occurrence of the dataset. Results show that the overall seismic rate (including mainshock and aftershock events) for events with local magnitudes ( M L ) above 2.5 has been escalated by a factor of more than 100, from 0.05 to more than five events per day, between January 1975 and August 2014. For this overall increase, the contribution of the mainshock events is estimated to be ~56%. Assuming the b -value of the Gutenberg–Richter law is 1.0, the probability of exceeding M L  5.0 in a 2-month period is predicted to have increased from about 0.05 to more than 0.5 during the study period. A sensitivity analysis is presented to show how the probabilistic inference is affected by the assumed b -value and the assumed maximum event magnitude. The proposed method can provide a statistical basis for quantitatively assessing the process of induced seismicity. In addition, it can be employed as a decision-support tool to identify areas with increasing levels of hazard and to guide strategies for risk mitigation.

Description: It is well documented that wastewater disposal by underground injection can increase seismic activity. However, it is challenging to predict whether and when this effect occurs, as it can manifest itself even many years after the beginning of disposal operations. Continuous instrumental monitoring of the seismic activity is a first step enabling early detection of symptoms of change, updating of site and regional risk assessment, and prompt response. To avoid misdetection, however, seismic records have to be rationally analyzed, properly modeling the statistical features of the earthquake occurrence process. In this study, we develop a statistical approach to detect increments in seismic rate, accounting for model uncertainty (which is particularly acute when the monitoring period is short) and interdependence among events. The approach is composed of two steps: (1) stochastic earthquake declustering identifies mainshocks, and (2) the hypothesis of a constant rate of mainshocks is statistically tested. The method is applied to the analysis of the Oklahoma injection region, demonstrating that it is able to detect an increment in seismic rate before the change is large enough to produce severe consequences. We also investigate the statistical power of the method using synthetic data simulated for a wide range of scenarios. We believe that the method can be employed as a preliminary decision-support tool to identify areas with trends in seismicity that require further investigation and to guide adaptive management and operation of disposal wells.

Description: 〈span〉〈div〉ABSTRACT〈/div〉We studied the problem of determining accurately the location of abandoned and sometimes undocumented wells and the challenging and increasingly important task related to subsurface reservoir integrity and regional economic development. We reviewed a variety of semiquantitative methods based on geophysical workflows, and we tested these with airborne magnetic data collected at two field sites. Our main conclusion is that airborne magnetic surveys represent a high-value tool to aid in the accurate determination of abandoned well locations and characteristics. At one site, two surveys were collected at slightly different altitudes to compare workflow robustness and allow the observed vertical magnetic gradient to be included in well detection workflows. We also investigated using focal zone anomaly statistics (using the magnetic field intensity and its first and second horizontal derivatives), analytic signal, tilt derivative, and calculated vertical gradient. In addition, we used a 3D inversion of a small subset of data to investigate the successful recovery of well-related magnetic susceptibility distribution and estimate subsurface well topology. The recovered magnetic susceptibility volume showed distinctive vertically elongated objects that correspond to known wells. Maximum likelihood estimation and confidence calculations were then applied to these data sets and indicated that high-confidence well locations could be determined and characterized using such airborne total magnetic data.〈/span〉

Description: Using an innovative workflow incorporating microseismic attributes and geomechanical well logs, we have defined major geomechanical drivers of microseismic expression to understand reservoir stimulation response in an engineering/geologic context. Microseismic data from a hydraulically fractured Marcellus well in the Appalachian Basin, central Pennsylvania, were sampled vertically through the event cloud, crossing shale, limestone, sandstone, and chert. We focused our analysis on the Devonian organic shale and created pseudologs of moment magnitude (M w ), b -value, and event count. The vertical moving-average sampling of microseismic data was completed such that the sample interval matched that of the geophysical well logs. This technique creates robust, high-resolution microseismic logs that show subtle changes in microseismic properties and allow direct crossplotting of microseismic versus geophysical logs. We chose five geomechanical properties to form the framework against which to interrogate the microseismic data: Young's modulus (YM), Poisson's ratio (PR), brittleness, lambda-rho, and mu-rho. Additionally, we included natural gamma as a useful measure of organic content. Having defined this microseismic-geomechanical crossplot space, we derived insights into the response of these units during hydraulic fracturing. Observations include: (1) larger magnitude microseismicity occurs in high PR rocks, and high event counts are found in low PR rocks; (2) low b -value (high in situ stress) is consistent with the occurrence of larger magnitude events and low event counts; and (3) YM and brittleness act as bounding conditions, creating "sweet spots" for high and low M w , event count, and stress. In our crossplot space, there is a meaningful link between microseismicity and the elastic properties of the host rock. In light of this dependence of stimulation potential on elastic properties, the calculation of microseismic pseudologs at stimulation sites and application of our crossplot framework for microseismic-geomechanical analysis in unconventional shale will inform operators in planning and forecasting stimulation and production, respectively.

Description: Hydraulic fracturing is a well-completion activity that enables the economic extraction of hydrocarbons from unconventional reservoirs such as shale, which are of a naturally low permeability. During hydraulic fracturing, the movement of water into the reservoir from newly created hydraulic fractures reactivates preexisting faults and fractures, initiating shear failure on a complex network of planar features. Although the energy released during hydraulic fracturing is small, the seismic emissions arising from shear failure can be detected using surface and borehole seismometers, and these emissions are an important diagnostic tool for evaluating the effectiveness of reservoir stimulation. To investigate seismic activity during hydraulic fracturing of six horizontal Marcellus Shale wells in Greene County, Pennsylvania, we deployed a surface seismometer within the footprint of the horizontal wells. We recorded 53 high-amplitude, impulsive events and 144 long-period, long-duration (LPLD) events. LPLD events identified in this study show a low-frequency, low-amplitude precursor followed by a high-frequency, high-amplitude primary S-wave signal and are similar to long-duration events (30–60 s) identified in previous studies. After a thorough investigation, we found no temporal correlation with seismic events reported in regional earthquake catalogs and data from USArray stations, suggesting that these observed LPLDs are not attenuated signals from regional earthquakes. Spectral analysis of LPLD events reveals concentrated energy between 1 and 30 Hz. LPLD events were found to occur most frequently when the pumping pressure and rate were at maximum values. Recent findings suggest that "slow-slip emission" along discontinuities that are unfavorably oriented in the ambient stress field is likely a dominant and vital mechanism of deformation during reservoir stimulation. We compared the radiated microseismic energy plus the theoretical fracture energy to the total hydraulic input energy and found an approximately 75% deficit in the energy budget. We propose that LPLD deformation accounts for some portion of this energy deficit.