Subsurface Fluid Migration Research Articles

Influence of Chemical Weathering and Microcracks on Permeability Variations in Crystalline Rocks

Rock permeability, an important factor in subsurface fluid migration, can be influenced by microcracks and chemical weathering due to water–rock interactions. Understanding the relationship between permeability, chemical weathering, and microcracks is crucial for assessing fluid flow in rocks. This study focuses on the hydrogeological characteristics of granite and gneiss, potential host rocks for high-level radioactive waste disposal in South Korea. Samples were analyzed for permeability, porosity, P-wave velocity, and chemical weathering indices. Regression analysis revealed a weak correlation between permeability and both porosity and rock density, while an inverse correlation was observed between permeability and chemical weathering indices. Interestingly, some samples showed low permeability (10−21 to 10−22 m2) despite high weathering, while others showed high permeability (10−18 to 10−19 m2) despite low weathering. SEM-EDS analysis indicated the presence of microcracks within the rocks or the filling of these cracks with secondary minerals. The findings suggest that chemical weathering generally increases pore size and porosity, but actual permeability can vary depending on the presence and connectivity of microcracks and the extent to which they are filled with secondary minerals. Therefore, both chemical weathering and microcrack connectivity must be considered when evaluating the hydrogeological characteristics of crystalline rocks.

Estimating pore pressure in tight sandstone gas reservoirs: A comprehensive approach integrating rock physics models and deep neural networks

Pore pressure serves as an important driving power for subsurface fluid migration and therefore has a significant impact on gas accumulation and enrichment in tight sandstone reservoirs. Tight gas is typically produced in overpressure regions, where pressure coefficients are notably elevated. Thus, it is crucial to establish an effective methodology for precise pore pressure estimation. This study introduces an approach to improve pore pressure prediction by incorporating rock physical modeling and deep neural networks (DNNs) into the classical Eaton method. Compared to conventional techniques relying on empirical correlations between pressure coefficients and elastic properties, the proposed method considers the influence of porosity, fluids, and lithology, which could enhance reliability in pore pressure prediction. Meanwhile, a prediction model is developed using logging data and DNNs to estimate mineralogical volumetric fractions based on elastic properties. This prediction model allows improved estimation of rock matrix elastic properties using seismic-inverted data, which is crucial for estimating normal compaction velocity to extend pore pressure prediction from individual boreholes to the whole study area. Real data applications demonstrate that the predicted pressure coefficients derived from seismic data using the method presented in this paper align well with the gas enrichment estimated in previous studies for the tight sandstone reservoirs. Furthermore, regions with high values of pressure coefficients correspond to high gas content. These findings validate the effectiveness of the proposed methodology, which can provide valuable insights for identifying potential tight sandstone reservoirs.

INFLUENCE OF PLEISTOCENE GLACIATION ON PETROLEUM SYSTEMS AND GAS HYDRATE STABILITY IN THE OLGA BASIN REGION, BARENTS SEA

This study presents the results of a 2D numerical basin and petroleum systems model of the Olga Basin in the NW Barents Sea offshore northern Norway, a frontier exploration area in which there are abundant seafloor oil and gas seepages. The effects of Pleistocene ice sheet advances on rock properties and subsurface fluid migration in this area, and on seafloor hydrocarbon seepage, are not well understood. The 2D numerical model takes account of recurrent ice advances and retreats, together with related erosional and temperature effects, and investigates the influence of these parameters on fluid migration. Model results show that Pleistocene glaciations reduced the temperature in the sedimentary succession in the Olga Basin by up to 20 °C, for example in the uppermost Cretaceous and Jurassic sediments which underlie the seafloor down to a depth of 0.5 to 1 km. The decrease in temperature was in general predominantly related to the intensity of glacial erosion, which was set in this study to a depth of 600 m based on previous studies. Hydrocarbon fluids expelled from potential thermogenic source rocks of Carboniferous to Triassic ages on the SW margin of the Olga Basin migrated to the seafloor through permeable carrier beds. However, fluid migration to the surface in the NE of the study area took place along fault conduits. In a closed fault model scenario, only 0.3 Mt of hydrocarbons are modelled to have migrated along the 0.5 km wide model section; in a second scenario with partially open faults, about 22 Mt of hydrocarbons, representing about 11% of the total hydrocarbons generated by potential thermogenic source rocks in the study area, were lost to the surface during the Pleistocene. The potential for microbial methane generation in the Olga Basin was limited both during the Pleistocene and at the present day due to the significant reduction in temperature during glacial episodes, and due to the intense glacial‐related erosion of the Mesozoic to Cenozoic stratigraphy. During glacial stages, the gas hydrate stability zone beneath the ice sheet is modelled to have extended to a depth of up to 900 m for a pure methane composition, and to a depth of up to 1100 m for a possible thermogenic‐sourced mixed gas composition of 90% methane, 7% propane and 3% ethane. Gas hydrates with this mixed composition are modelled to have been stable in the Olga Basin during the last three glacial advances and into the present. These modelling results provide an insight into the key factors controlling the migration and surface leakage of hydrocarbon fluids in the Olga Basin region, and into the effects of glaciations on rock properties in a glaciated basin.

Structural and stratigraphic controls on reservoir architecture: A case study from the lower Oligocene Vicksburg Formation, Brooks County, Texas

A detailed structural and stratigraphic framework provides a precise understanding of the controls on the vertical and spatial distribution of reservoir units, as well as the identification of the most prospective hydrocarbon accumulation. In southern Brooks County, Texas, the Lower Oligocene Vicksburg Formation (LOVF), is being influenced by the Vicksburg Fault Zone (VFZ). This zone is characterized by listric-normal faults that have formed highly faulted rollover anticlines, which are valuable structural traps for hydrocarbon exploration. This study integrated 568 3D seismic lines, 39 well logs, and three velocity surveys to build a comprehensive conceptual model that explores how secondary synthetic (dipping east), antithetic (dipping west), and Texas Gulf Coast-perpendicular faults are influencing accumulation and spatial distribution of hydrocarbons within the LOVF (La Rucias Field). Results indicate that synthetic, antithetic, and coast-perpendicular faults affecting the V-102, V-17, and V-19 horizons provide conduits for hydrocarbon migration. Antithetic faults and coast-perpendicular faults within the rollover anticline are terminating beneath an overlying shale seal layer between the V-16 and V-17, influencing economic hydrocarbon accumulations through multiple reservoirs. While synthetic faults affect the overlying seal layer migrating hydrocarbon out of the V-102, V-17, and V-19, bidirectional faulting linking antithetic and coast-perpendicular faults provide additional pathways for hydrocarbon accumulation. Spatial distribution of hydrocarbons varies with the targeted horizon. V-102 reservoirs are situated on the western flank of the rollover anticline, whereas the V-17 and V-19 reservoirs are located on structural highs where overlying shale seal layer remains unaffected by the antithetic faults. V-17 and V-19 reservoirs are influenced by bidirectional faulting that terminates beneath the shale seal layer, enabling the accumulation within the rollover anticline. Petrophysical and lithological investigations of the seal layers within the V-102, V-17 and V-19 are highly recommended for better understanding of vertical compartmentalization of reservoir intervals and accumulation estimates within the La Rucias Field. Investigating the control of these fault systems enhances our understanding of subsurface fluid migration and accumulations (oil, gas, groundwater, and contaminants) in the expanded Vicksburg productivity trends. Furthermore, the insights gained from the examination of these fault systems not only contribute to our comprehension of subsurface fluid migrations within the Vicksburg trends but also serve as a valuable reference for similar geological systems and reservoirs worldwide, ultimately aiding in the development of more effective resource management.

Subsurface fluid flow: A case study from the Indo-Gangetic peripheral foreland basin

Subsurface fluid flow involves migration of fluids from source to surface through a wide range of geologic structures, and thus plays a crucial role in unlocking potential hydrocarbon plays. Such studies have been mainly carried out in the marine environment by means of high-resolution seismic data in unravelling the fluid flow system and remains poorly documented for on-land data. This research attempts to explore the fluid flow activity in onshore petroliferous Indo-Gangetic peripheral foreland basin of India by using high-quality 3D seismic reflection data. Several seismic attributes have been utilised for efficiently describing the patterns of subsurface fluid flow structures. The attribute-responses of fluid flow are fused into a single attribute, called the Fluid Cube meta-attribute, by designing a workflow based on artificial neural network, which has enabled to elucidate the subsurface fluid migration routes. The subsurface fluid flow features are imaged as relatively vertical mounded structure with conical vent-like morphology. Internally, these features are associated with moderately distorted reflections. The reflections at the top are vertically stacked with medium to high amplitudes. The Fluid Cube meta-attribute demonstrates that subsurface fluid vertically migrates from the Neoproterozoic strata through minute fracture networks and weaker strata of the overlying Tertiary succession. The analysis of surficial geochemical anomalies corroborates quite reasonably with these observations. Thus, the Ganga peripheral foreland basin could be considered a promising area with potential leads that can be unlocked for hydrocarbon exploration. The analysis presented through this research could be efficiently carried out over other onshore basins worldwide.

Exploring seismic detection and resolution thresholds of fault zones and gas seeps in the shallow subsurface using seismic modelling

Seismic resolution and illumination issues are sources of challenges in the detailed imaging and detection of subsurface fault architecture and fluid migration. Improved constraints on resolution can provide input into monitoring requirements and detectability of CO2 leakage, and fault-sealing properties during subsurface visualization of migration pathways. This study explores detection and resolution thresholds via synthetic seismic imaging of a detailed shallow normal fault model with realistic fault architecture including sub-seismic structures of damage zones. The base fault model is built from interpretations of high-resolution P-Cable seismic data and is further developed and conditioned by outcrop-based observations and empirical laws for fracture and deformation band distribution. Damage zones of sandstone layers host deformation bands contrary to shale layers with fractures, while mixed lithologies (shaley sandstone and sandy shale) are subjected to a combination of the two deformation mechanisms. We utilize a 2D point-spread function based convolution seismic modelling to produce the synthetic seismic images. Test scenarios include one baseline fault model without a damage zone (M1), and five more advanced/detailed fault models incorporating features known from outcrops (M2-M6; damage zones, an isolated fracture corridor, and gas seeps (CO2) along damage zones of faults and in the fracture corridor). Furthermore, sensitivity analyses on two selected models test the effect of changing the illumination angle and wavelet. The results show that: (1) faults with damage zones have larger disturbances in seismic signals than faults without damage zones, (2) stronger amplitudes are distinguished for models with CO2-filled fractures, (3) a fracture corridor (5 m at it widest) is clearly visible where it crosses horizons bounding horizontal layers, (4) sensitivity tests show good imaging for illumination ≥45°, which is the average dip of the main fault segment, and (5) learnings from fault modelling offer guidance for seismic monitoring.

Fundamental constraints of lithologically-controlled fault networks on gas migration and accumulation for fractured carbonates in the western Sichuan Basin, China

The structure of subsurface fault networks, which has a significant effect on hydrocarbon migration, is inherently difficult to recognize and map. 3D seismic surveys provide an opportunity to map the fault system within a carbonate reservoir of Xinchang in the western Sichuan Basin, China. We calculated attributes from the seismic reflection data that covers an area of 1330 km2, including variance, edge detection, dip-magnitude and dip azimuth. We focused on the Triassic Leikoupo Formation of dolostone, at ∼6 km depth, adjacent to the Longmen Shan thrusting range. The mapped attributes display (1) a primary network of faults in an orthogonal pattern in the anticline, striking East-West and North-South, and (2) a secondary, conjugate fault set in the fold limb, striking Northeast and Northwest with a length range of 1–10 km. The latter set of conjugate faults are confined to a 300 m-thick layer of dolostone, diminishing into underlaying unit of anhydrite. We envision that the conjugate fault networks and associated fractures enhance gas charging and reflect a model of self-sourced migration and accumulation in the Leikoupo Formation. These results are significant for sweet-spot evaluation of carbonate reservoirs in the Sichuan Basin, and provide insights for understanding the migration of subsurface fluids.

Feasibility of using the P-Cable high-resolution 3D seismic system in detecting and monitoring CO2 leakage

The P-Cable technology is an acquisition principle for high-resolution and ultra-high-resolution 3D seismic data. Many 3D seismic datasets have been acquired over the last decade, but the application in time-lapse studies for monitoring of CO2 storage is a new and intriguing topic. High-resolution 3D (HR3D) seismic has the potential to detect and monitor CO2 leakage at carbon capture and storage sites with higher accuracy at depths ∼0−2 km below the seafloor compared to more traditional conventional seismic time-lapse data. Here, we synthesize and evaluate research on detection of subsurface CO2 movement using the P-Cable system and address the comparative advantages and disadvantages of conventional and HR3D technologies for subsurface fluid migration monitoring. Studies on P-Cable 4D seismic data show good repeatability (NRMS, 10–40 %), indicating a future monitoring potential. Analysis of detection limits of CO2 data from a CO2 storage site show the ability to detect very small amounts of CO2 (1.3–10.6 t; 3.3–27.4 % gas saturation) in the shallow subsurface. These detection limits are ∼30−300 times smaller than the detection limits of conventional seismic data at similar depths. We conclude that the P-Cable acquisition system can be a valuable monitoring tool in detecting small leakages and can complement conventional seismic data monitoring of the deeper interval.

Classification of and changes within formation water types in Danish North Sea Chalk: A study of the Halfdan, Dan, Kraka and Valdemar oil reservoirs

The ionic composition of produced water from four oil producing chalk fields in the Danish North Sea has been interpreted to shed light on the temporal and regional variations in the water composition. The data includes 8749 samples, covering 157 wells, analyzed for eight ions. The data dates back to the start of production (1972–2007) and the most recent data is from 2015. The variations are viewed in a regional context with respect to subsurface geology, fluid migration and production strategy since parts of the fields have been extensively water flooded by injection of seawater. We present results from both time series analysis and from multivariate statistical methods and show that produced water across the four fields, up to 55 km apart, show gradual changes in the composition reflecting regional scale hydrodynamics as well as subsurface geology. We use multivariate statistical analysis to extract end-member compositions to facilitate comparison between basins and for an evaluation of scale and corrosion risk. The data analysis shows five main water types. These are mainly differentiated by salinity and the concentrations of the divalent cations. The study is discussed in the context of examination of interconnection between close-lying fields including aquifer support and distribution of principal ions associated with corrosion and scale formation.

Multiscale fracture and damage zone characterization in a tight sandstone reservoir, Sichuan Basin, China

Fractures within fault damage zones are crucial for the migration of subsurface fluids, which is challenging the characterization of the fractures and damage zones in the subsurface due to the lack of subsurface data. We have investigated the fractures and fault damage zones in the tight sandstone gas-bearing Xujiahe Formation in the northeast Sichuan Basin, China, based on a comprehensive study of seismic data, well log, core, as well as field observations. We have demonstrated the structure and distribution of damage zones from the basin-scale down to the microscale. The core samples find mostly opening-mode microfractures that can be subcritically generated under low tectonic stress fields since the Late Triassic-Middle Jurassic. These opening-mode microfractures in such ultra-low-permeability sandstone are likely to provide a storage volume for coal gas to accumulate. Macrofractures from image logging and outcrop display well-developed joint networks within the sandstone. The basin-wide distribution of such macrofractures implies potential conduits for gas migration during basin uplift in the Cretaceous. The damage zones are formed and controlled by a system of reverse faults, with thicknesses ranging 100–1500 m. The multiscale analysis of fractures implies that, for the tight sandstone within the Xujiahe Formation, the fractures and damage zones are likely to control the gas migration and accumulation during the tectonic transformation from the Late Cretaceous to the Quaternary Period. The overpressure history, coupled with the fracture system, enhances the redistribution of the gas reservoir. This approach and these data lead to a more in-depth understanding of the fractures and damage zones on a regional scale, which could extend to hydraulic and mechanical characterization of damage zones in the upper crust.

Analysis of fault damage zones using three-dimensional seismic coherence in the Anadarko Basin, Oklahoma

Fault damage zones may significantly affect subsurface fluid migration and the development of unconventional resources. Most analyses of fault damage zones are based on direct field observations, and we expand these analyses to the subsurface by investigating the damage zone structure of an approximately 32-km (∼105-ft)-long right-lateral strike-slip fault in Oklahoma. We used the three-dimensional (3-D) seismic attribute of coherence to first define its regional and background levels, and then we evaluated the damage zone dimensions at multiple sites. We found damage zone thickness of approximately 1600 m (∼5300 ft) at a segment that is dominated by subsidiary faults, and it is slightly thicker at a segment with a pull-apart basin. The damage zone intensity decays exponentially with distance from the fault core, in agreement with field observations and distribution of seismic events. The coherence map displays a strong asymmetry of the damage zone between the two sides of the 3-D fault, which is related to the subsidiary structures of the fault zone. We discuss the effects of heterogeneous stress field on damage zone evolution through the detected subsidiary structures. It appears that seismic coherence is an effective tool for subsurface characterization of fault damage zones.

Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs

Abstract The geological processes that create fluid storage capacity and connectivity in global fractured basement reservoirs are poorly understood compared to conventional hydrocarbon plays. Hosting potentially multibillion barrels of oil, the upfaulted Precambrian basement of the Rona Ridge, offshore west of Shetland, UK, gives key insights into how such reservoirs form. Oil presence is everywhere associated with sub-millimeter- to meter-thickness mineralized fracture systems cutting both basement and local preseal cover sequences. Mineral textures and fluid inclusion geothermometry suggest a low-temperature (90–220 °C), near-surface hydrothermal system, as does the preservation of clastic sediments in the same fractures. These fills act as permanent props holding fractures open, forming long-term fissures in the basement that permit oil ingress and storage. Calcite-fill U-Pb dating constrains the onset of mineralization and contemporaneous oil charge to the Late Cretaceous. The additional preservation of oil-stained injected sediment slurries and silica gels along basement faults suggests that rift-related seismogenic faulting initiated lateral oil migration from Jurassic source rocks into the adjacent upfaulted ridge. Subsidence below sea level in the latest Cretaceous sealed the ridge with shales, and buoyancy-driven migration of oil into the preexisting propped fracture systems continued long after the cessation of rifting. These new observations provide an explanation for the viability of sub-unconformity fractured basement reservoirs worldwide, and have wider implications for subsurface fluid migration processes generally.

Geospatial Statistics Elucidate Competing Geological Controls on Natural CO2 Seeps in Italy

Site selection for the geological storage of CO2 for long timespans requires an understanding of the controls on containment, migration, and surface seepage of subsurface CO2 fluids. Evidence of natural CO2 migration from depth to the surface is documented at 270 sites from Italy, a prolific CO2 province. Previous studies indicate that CO2 delivery to and from buried structures that host CO2 accumulations is fault controlled but competing controls on the CO2 flow pathways affect the location and style of CO2 release. Here, we conduct a meta-analysis using a novel geospatial approach to statistically determine the relationship between the geological setting and structures and the CO2 seep spatial distribution and characteristics (morphological type, flux, and temperature) in Central Italy. We find that seep distribution differs on two spatial scales corresponding to the geological setting. On large scales (>5 km), seeps are isotropically distributed and align with regional structures such as anticlines, decollements, and extensional faults. On local scales (<5 km), seeps cluster and align with subsidiary geologic structures, including faults and lithological boundaries. The detailed location and flux of seeps within clusters are influenced by the regional structural domain: in the Tyrrhenian, seeps tend to be located along fault traces, whereas seeps are located as springs in the tip and ramp regions of fault scarps in the Apennines. Thus, our geospatial approach evidences, at a regional scale, how macrocrustal fluid flow is governed by deep extensional and compressional features but once CO2 reaches shallower structures, it evidences how smaller scale features and hydrogeological factors distribute the CO2 fluids in the near surface, dependent on the geological setting. This work not only demonstrates useful application of a novel geospatial approach to characterize competing crustal controls on CO2 flow at different scales but also informs the design of appropriate site characterization and surface monitoring programs at engineered carbon stores.

Biomineralization and Wellbore Integrity: A Microscopic Solution to Subsurface Fluid Migration

Biomineralization and Wellbore Integrity: A Microscopic Solution to Subsurface Fluid Migration

Assessment of ground deformation following Tenerife’s 2004 volcanic unrest (Canary Islands)

After the 2004 unrest on Tenerife (Canary Islands), the Global Navigation Satellite System (GNSS) network called TEGETEIDE was designed and deployed with seven survey mode stations to contribute to a volcanic alert system. Starting in 2008, public access data from continuous GNSS stations, managed by public institutions, were included in this network allowing measurement of ground displacement at 14 locations in Tenerife. Data acquired from 2005 to 2015 was analysed to assess ground deformation in Tenerife following the 2004 volcanic unrest. The overall ground deformation depicted compression in the central Las Cañadas Caldera possibly caused by gravitational subsidence of the high central volcanic cone. This sinking is generalised around the whole island but is less in the northeast (Anaga Massif) where we found an extension rate close to 200 strain/y that could be related to a secondary submarine fault accommodating rifting to the northeast and isolating the behaviour of this massif. At the south volcanic field (south rift), another localized area with an extensional deformation was detected, possibly resulting from the subsurface fluid migration or mass addition that caused the 2004 volcanic unrest because is located following the seismic swarm alignment along Icod Valley towards Roque del Conde massif that persists since that event. We also detected residual plate velocity indicating movement of Tenerife towards Gran Canaria that should be studied in the context of the entire Canary Islands archipelago.

Fluctuations in populations of subsurface methane oxidizers in coordination with changes in electron acceptor availability

The concentrations of electron donors and acceptors in the terrestrial subsurface biosphere fluctuate due to migration and mixing of subsurface fluids, but the mechanisms and rates at which microbial communities respond to these changes are largely unknown. Subsurface microbial communities exhibit long cellular turnover times and are often considered relatively static-generating just enough ATP for cellular maintenance. Here, we investigated how subsurface populations of CH4 oxidizers respond to changes in electron acceptor availability by monitoring the biological and geochemical composition in a 1339 m-below-land-surface (mbls) fluid-filled fracture over the course of both longer (2.5 year) and shorter (2-week) time scales. Using a combination of metagenomic, metatranscriptomic, and metaproteomic analyses, we observe that the CH4 oxidizers within the subsurface microbial community change in coordination with electron acceptor availability over time. We then validate these findings through a series of 13C-CH4 laboratory incubation experiments, highlighting a connection between composition of subsurface CH4 oxidizing communities and electron acceptor availability.

Calcite veins as an indicator of fracture dilatancy and connectivity during strike-slip faulting in Toarcian shale (Tournemire tunnel, Southern France)

The reactivation of faults induced by natural/human induced fluid pressure increases is a major concern to explain subsurface fluid migration and to estimate the risk of losing the integrity of reservoir/seal systems. This study focusses on paleo-fluid migration in a strike slip fault with >100 m long, affecting a Toarcian shale (Causses Basin, France). A high calcite concentration is observed in a 5 cm thick zone at the boundary between the fault core and damage zone. Cumulated displacements in this zone are of millimeter-to-centimeter-scale offsets and different dilatant deformation textures are observed. The zone is affected by thin slip planes containing gouge. Cathodo-luminescence observations indicate that two phases of vein formation occurred. The first phase coincides with the fluid migration along this centimeter thick dilatant zone. The second one is associated to re-shear along the millimeter thick slip planes that results in more localized mineralization, but also in a better hydrologic connection through the shale formation. These results show that in shales fluids may migrate off a slipping surface in centimeter scale dilatant volumes, at first controlled by the intact shale anisotropy related to bedding and then favored by brecciating, structures re-orientation and strengthening processes induced by calcite sealing effects.

Episodic massive mud eruptions from submarine mud volcanoes examined through topographical signatures

The role of mud volcanism on subsurface fluid migration and material cycling has long been debated. Here we compile the heights and radii of offshore mud volcanoes and estimate a mean volume of episodic massive mud eruptions based on previous studies into granular flows. The volume is estimated as a function of the ratio of height to basal radius of the mud volcano's body under reasonable assumptions of the sizes of the mud conduit. Nearly all known offshore mud volcanoes are found to be polygenetic with the mean individual eruption volume of the pie-type mud volcano being several orders of magnitude larger than that of the cone type. The frequent occurrence of pie-type mud volcanoes in accretionary margins characterized by high-sediment influx is explained by their efficiency in the transport of large amounts of fluidized sediments from deep depths to the seafloor.

Effective Permeabilities of Abandoned Oil and Gas Wells: Analysis of Data from Pennsylvania

Abandoned oil and gas (AOG) wells can provide pathways for subsurface fluid migration, which can lead to groundwater contamination and gas emissions to the atmosphere. Little is known about the millions of AOG wells in the U.S. and abroad. Recently, we acquired data on methane emissions from 42 plugged and unplugged AOG wells in five different counties across western Pennsylvania. We used historical documents to estimate well depths and used these depths with the emissions data to estimate the wells' effective permeabilities, which capture the combined effects of all leakage pathways within and around the wellbores. We find effective permeabilities to range from 10(-6) to 10(2) millidarcies, which are within the range of previous estimates. The effective permeability data presented here provide perspective on older AOG wells and are valuable when considering the leakage potential of AOG wells in a wide range of applications, including geologic storage of carbon dioxide, natural gas storage, and oil and gas development.

Effects of episodic fluid flow on hydrocarbon migration in the Newport‐Inglewood Fault Zone, Southern California

AbstractFault permeability may vary through time due to tectonic deformations, transients in pore pressure and effective stress, and mineralization associated with water‐rock reactions. Time‐varying permeability will affect subsurface fluid migration rates and patterns of petroleum accumulation in densely faulted sedimentary basins such as those associated with the borderland basins of Southern California. This study explores the petroleum fluid dynamics of this migration. As a multiphase flow and petroleum migration case study on the role of faults, computational models for both episodic and continuous hydrocarbon migration are constructed to investigate large‐scale fluid flow and petroleum accumulation along a northern section of the Newport‐Inglewood fault zone in the Los Angeles basin, Southern California. The numerical code solves the governing equations for oil, water, and heat transport in heterogeneous and anisotropic geologic cross sections but neglects flow in the third dimension for practical applications. Our numerical results suggest that fault permeability and fluid pressure fluctuations are crucial factors for distributing hydrocarbon accumulations associated with fault zones, and they also play important roles in controlling the geologic timing for reservoir filling. Episodic flow appears to enhance hydrocarbon accumulation more strongly by enabling stepwise build‐up in oil saturation in adjacent sedimentary formations due to temporally high pore pressure and high permeability caused by periodic fault rupture. Under assumptions that fault permeability fluctuate within the range of 1–1000 millidarcys (10−15–10−12 m2) and fault pressures fluctuate within 10–80% of overpressure ratio, the estimated oil volume in the Inglewood oil field (approximately 450 million barrels oil equivalent) can be accumulated in about 24 000 years, assuming a seismically induced fluid flow event occurs every 2000 years. This episodic petroleum migration model could be more geologically important than a continuous‐flow model, when considering the observed patterns of hydrocarbons and seismically active tectonic setting of the Los Angeles basin.