Structure Segmentation and Transfer Faults in the Marcellus Shale, Clearfield County, Pennsylvania: Implications for Gas Recovery Efficiency and Risk Assessment Using 3D Seismic Attribute Analysis

The Marcellus Shale has become an important unconventional gas reservoir in the oil and gas industry. Fractures within this organic-rich black shale serve as an important component of porosity and permeability useful in enhancing production. Horizontal drilling is the primary approach for extracting hydrocarbons in the Marcellus Shale. Typically, wells are drilled perpendicular to natural fractures in an attempt to intersect fractures for effective hydraulic stimulation. If the fractures are contained within the shale, then hydraulic fracturing can enhance permeability by further breaking the already weakened rock. However, natural fractures can affect hydraulic stimulations by absorbing and/or redirecting the energy away from the wellbore, causing a decreased efficiency in gas recovery, as has been the case for the Clearfield County, Pennsylvania study area. Estimating appropriate distances away from faults and fractures, which may limit hydrocarbon recovery, is essential to reducing the risk of injection fluid migration along these faults. In an attempt to mitigate the negative influences of natural fractures on hydrocarbon extraction within the Marcellus Shale, fractures were analyzed through the aid of both traditional and advanced seismic attributes including variance, curvature, ant tracking, and waveform model regression. Through the integration of well log interpretations and seismic data, a detailed assessment of structural discontinuities that may decrease the recovery efficiency of hydrocarbons was conducted. High-quality 3D seismic data in Central Pennsylvania show regional folds and thrusts above the major detachment interval of the Salina Salt. In addition to the regional detachment folds and thrusts, cross-regional, northwest-trending lineaments were mapped. These lineaments may pose a threat to hydrocarbon productivity and recovery efficiency due to faults and fractures acting as paths of least resistance for induced hydraulic stimulation fluids. These lineaments may represent major transfer faults that serve as pathways for hydraulic fluid migration. Detection and evaluation of fracture orientation and intensity and emphasis on the relationship between fracture intensity and production potential is of high interest in the study area as it entails significant time and cost implications for both conventional and unconventional hydrocarbon exploration and production

QUANTITATIVE ANALYSIS OF ANOMALOUS SEISMIC AMPLITUDES CAUSED BY FLUID MIGRATION

Two- (2D) and three- (3D) dimensional pre-stack and post-stack seismic reflection data are used to investigate the processes which have led to the development of amplitude anomalies on reflections in the faulted, Cenozoic overburden on the Laminaria High, Northwest Shelf of Australia. The integration of amplitude and seismic attribute maps for four key horizons (the seabed, Horizon H9, Horizon H10 and Horizon H13) with the corresponding two-way time (TWT) structure maps has identified the structural controls on the distribution of seismic anomalies. On the seabed, the main anomaly is located on the up-dip side of the fault trace, and is elongated parallel to the local time structure contours. These observations are consistent with the anomalies having developed in response to structurally-controlled fluid seepage along, and up-dip migration away from the fault trace. Amplitude anomalies associated with the deeper H9 reflector are also located adjacent to fault traces but are discordant to the local time structure contours. This observation suggests that the anomalies may be due to cemented hardgrounds that formed due to seepage when the faults intersected the palaeo-seafloor but were subsequently buried and deformed during ongoing sedimentation and fault growth/linkage. Reprocessing of the 2D and 3D seismic pre-stack data supports the seismic interpretation of amplitude anomalies at the seabed. It is concluded that these anomalies are robust – that is, they are likely to reflect geological processes and are not simply a function of the chosen seismic processing workflow – and are caused by localised changes in acoustic impedence in the subsurface. More important is that using processed data without the knowledge of the background processing sequence for the data could be an issue in any 2D or 3D seismic interpretation. For this reason the veracity of processing of any seismic data needs to be questioned, and should not be taken for granted especially if different surveys produce conflicting interpretations. 2D hydrocarbon migration modelling combined with fault slip- and dilation-tendency analyses were undertaken in order to investigate the impact of faults and host-rock lithologies on hydrocarbon seepage at the present-day sea floor. Results show that some active faults associated with amplitude anomalies (e.g. Fault F10) are critically stressed, assuming a static, and spatially homogeneous regional stress field. However, other faults associated with amplitude anomalies (e.g. Fault F11) appear not to be critically stressed. These results suggests that the “regional” stress field could, in fact, vary spatially and temporally allowing faults in different parts of the study area to become critically stressed – hence act as fluid migration pathways – at different times. The migration models show that hydrocarbon migration pathways are strongly influenced by fault-zone properties, specifically the capillary entry pressure (CEP) along faults. The dip of the sediment layers also influences hydrocarbon leakage from the subsurface to the seabed. In general, the migration models show vertical hydrocarbon migration along faults coupled with lateral migration below the seal layers and between faults. Fluids migrate along faults with two patterns of flow based on the CEP values along the faults: 1) focused – fluids migrate as a linear pattern along faults when the capillary entry pressure along the fault is within the lower range of the “background” CEP values; 2) diffuse – fluids are guided by faults when the capillary entry pressure along the fault is within the higher range of the “background” CEP values

Remote Sensing and GIS Analysis of Spatial Distribution of Fracture Patterns in the Makran Accretionary Prism, Southeast Iran

This study shows that remote sensing and GIS are powerful tools in identifying geologically induced lineaments from digitally enhanced ETM+ satellite imageries and the digital elevation model (DEM) in remote areas such as the Makran accretionary prism, southeast Iran. The presence of the conjugate shear fractures in the eastern part, along with the extensional, and the presence of reidal sets associated with the subsidiary fractures of the Minab-Zendal fault system in the western part, suggests that the structural pattern changes from pure shear to simple shear from east to the west across the prism. Moreover, the gradual increase in the value of the angle between the two conjugate shear fractures, from south (coastal Makran) to north across the prism, and the presence of high-angle north-dipping reverse faults, with few south-dipping normal faults, suggest that deformation changes from brittle, in the south, to ductile in the northern part of the prism

Rock physics changes due to CO2 injection : the CO2CRC Otway Project

The CO2CRC Otway Project aims to demonstrate that CO2 can be safely stored in a depleted gas field and that an appropriate monitoring strategy can be deployed to verify its containment. The project commenced in 2005, with the baseline 3D seismic collected early in January 2008. CO2 was injected into depleted gas reservoir known as Waarre-C at Naylor field in April 2008. The first monitor survey was recorded in January 2009, shortly after the injection of 35,000 tonnes of CO2. Early predictions in the program suggested that the resulting time-lapse seismic effect will be very subtle because of the reservoir depth, small area, complexity, small amount of CO2/CH4 in 80/20 ratio injected and most of all partial saturation of the reservoir sand. The key challenge than presented to this research was how subtle exactly is the effect going to be? To answer that question I had to develop a workflow that will produce very accurate prediction of the elastic property changes in the reservoir caused by CO2 injection. Then the sensitivity of time-lapse seismic methodology in detecting subtle changes in the reservoir is investigated.The rock physics model I propose uses the “effective” grain bulk modulus (Kgrain) to represent the average mineralogy of the grains. The validity of this approach is confirmed by good agreement achieved between Vpsat core with Vpsat computed from the log data using the “effective” modulus. . The use of “effective” Kgrain was further justified by petrographic analysis. This has increased the modelling precision and changed the predicted time-lapse effect due to CO2 injection from 3% as an average over the reservoir sequence as previously computed to nearly 6%. The significance is that 6% change could be detected with high precision monitoring methodologies. The in-situ saturation type is homogeneous, according to the analysis path assumed in this thesis. If some patchiness exists in the reservoir it will be away from the wells and it would further elevate CO2 related seismic effect.The time-lapse seismic methodology at Otway site utilised very high survey density in order to increase sensitivity. On the negative side, weak sources and the change of the source type between the surveys resulted in non-repeatability greater or of the similar order as the time-lapse signal were expected to be. Hence the interpretation of the time-lapse P-wave seismic data assumed somewhat different path. I used the model-based post-stack seismic acoustic inversion in a similar way that history matching is used in reservoir simulation studies. I performed successive fluid substitutions, followed by the well ties and inversions. The objective was to look into the inversion error. Then the modelled fluid saturation case that result in minimal inversion error reflects the most likely state of the reservoir. Modelling using 35,000 tonnes of CO2/CH4 mix with 35% water saturation and 65% CO2/CH4 mix produced the smallest error when reinstating logs to the 2009 reservoir state.The time-lapse anomaly observed in the data exceeds predictions derived through the rock physics model, seismic modelling and simulation models. This is likely to be the case in general as the effect of CO2 onto a reservoir is difficult to predict. A “conservative” approach may result in an under-prediction of time-lapse seismic effects. Consequently, the predicted and measured seismic effects can be used as the lower and the upper bound of the time-lapse effects at Naylor field, respectively. The method presented here for analysis of a subtle time-lapse signal could be applied to the cases with similar challenges elsewhere

Use of high resolution 3D seismic data to evaluate Quaternary valley evolution history during transgression, offshore San Luis Pass, Gulf of Mexico

textA novel, shallow-investigation, high-resolution 3D (HR3D) seismic acquisition system has been employed, for the first time in the Gulf of Mexico, to characterize CO₂ storage potential and de-risk targets for sequestration. HR3D data can image detailed depositional, architectural, and structural features in the shallow subsurface that have previously been below seismic resolution and/or excluded from industry surveys, which are optimized for deeper targets. One HR3D survey was collected in 2013 offshore San Luis Pass, TX and covers an area of 31.5 km². The dataset images the upper 500 meters of stratigraphy with unprecedented detail -- peak frequency of approximately 150Hz (eight 25m cables, spaced at 12.5m, 6.25m by 6.25m bin size). Imaged within this dataset at ~100ms TWTT, is a mappable erosional unconformity that is interpreted to be associated with the Brazos River system during the ~130ka glacial-eustatic lowstand and following transgression. Through the analysis of horizon slices and the geometries of the valley form and its dendritic features, the evolution history of the valley system during a transgressive episode can be characterized. Observations indicate that the system evolves from a lowstand meandering channel system with clear point-bar deposits to a transgressive estuary characterized by dendritic erosional features that is eventually flooded. These 3D data represent an exceptional example of a lowstand to transgressive transition and the sedimentary processes and architectures that characterize each interval. A seismically discontinuous zone is observed within the HR3D volume that is interpreted to be a gas chimney system emanating from a tested dry, 3-way structure in the lower Miocene (1.5km depth). Within the shallowest intervals (<100m) and at the top of the chimney zone, seismic attribute analysis reveals several high amplitude anomalies that are predominantly located within interpreted interfluvial zones. The anomalies fit into our stratigraphic and structural interpretation of the interval, in that they appear to sit at local structural, fault bounded highs within deposits interpreted to be coarser grained and are overlain by finer grained, transgressive deposits. These observations support the interpretation of these amplitude anomalies as shallow gas accumulations derived from a deeper, depleted gas reservoir. Interestingly, point-bar deposits as well as channel scour deposits within the same interval show no sign of charge, suggesting that these are either isolated from the migration flowpath, or too fine-grained to host significant saturations.Geological Science

Application of vertical seismic profiling for the characterisation of hard rock

Seismic imaging in hard rock environments is gaining wider acceptance as a mineral exploration technique and as a mine-planning tool. However, the seismic images generated from hard rock targets are complex due to high rock velocities, low contrasts in elastic rock properties, fractionated geology, complicated steep dipping structures and mineralogical alterations. In order to comprehend the complexity and utilise seismic images for structural mapping and rock characterisation, it is essential to correlate these images to known geology. An ideal tool for this purpose is Vertical Seismic Profiling or VSP. The VSP method can provide not only a means to correlate seismic images to geology but also to study the properties of the transmitted seismic field as it is modified by different rock formations, the origin of the reflected events and the corresponding reflector geometry. However, the VSP technique is rarely used in hard rock environments because of the cost and operational issues related to using clamping geophones in exploration boreholes, which are 96 mm or less in diameter. Consequently the main objective of this research is to produce an efficient VSP methodology that can be readily deployed for mineral exploration.An alternative to the clamping geophone is the hydrophone. Hydrophones are suspended in, and acoustically coupled to the borehole wall through, the borehole fluid. Borehole acoustic modes known as "tube-waves" are generated by seismic body waves passing the water column and are guided in the borehole due to the high acoustic impedance contrast between the rock and fluid. Tube-waves are 1-2 orders in magnitude higher in amplitude than seismic signal and mask reflected energy in hydrophone VSP profiles. As such the use of borehole hydrophone arrays to date has been restricted to direct body wave measurements only. I have effectively mitigated tube-waves in hydrophone VSP surveys with specific acquisition methodologies and refined signal processing techniques. The success of wavefield separation of tubewaves from hydrophone data depends critically upon; having high signal to noise ratio, well sampled data, pre-conditioning of the field data and processing in the field record (FFID) domain. Improvements in data quality through the use of high viscosity drilling fluids and baffle systems have been tested and developed. The increased signal to noise ratio and suppression of tube-wave energy through these technologies greatly enhances the performance of hydrophone VSP imaging.Non-standard wavefield separation techniques successfully removed strong coherent tube-wave noise. The additional wavefield separation steps required to remove high amplitude tube-waves does degrade the overall result with some fidelity and coherency being lost. However, a direct comparison of hydrophone and borehole clamping geophone VSP surveys has been conducted in the Kambalda nickel district and the two methodologies produced comparable results. The difference was that the hydrophone data were collected in a fraction of the time compared to clamping geophone equipment with significantly less risk of equipment loss and with reduced cost.The results of these field experiments and the data processing methodology used, demonstrate the potential of hydrophone VSP surveys in the small diameter boreholes typical of hard rock exploration. Thus, these results show that hydrophone VSP is a viable, cost effective and efficient solution that should be employed more routinely in hard rock environments in order to enhance the value of the surface seismic datasets being acquired