Barrier For Fluid Flow Research Articles

Progressive development of stylolites in cryptocrystalline quartz

Stylolites are common dissolution structures in carbonate rocks in the upper crust and document a coupled physical-chemical feedback during pressure solution in rocks. By applying high-resolution electron-based analysis we investigate a rare example of stylolites in cryptocrystalline quartz (novaculite) from western Sichuan Basin, China. The investigated stylolites exhibit an increased amplitude from the tip toward the center, which correlates with the maturity of stylolites and thus provides an excellent opportunity to infer the dynamic processes operating during stylolite formation. Microstructurally, stylolites are characterized by a concentration of biotite and hematite, dissolving irregular quartz grains, and the occurrence of dissolution porosity. By investigating the progressive co-variations between stylolite amplitude and microstructure characteristics from the stylolite tip toward the center, we demonstrate, for the first time, the evidence of stylolite growth through anticracks. It is inferred that stylolites in cryptocrystalline quartz originate from the propagation of mode I micro-cracks around pre-existing biotite, followed by pore-scale dissolution. Both processes, micro-cracking and dissolution, are interacting and co-evolving via a self-organization fashion during shortening. Our study contributes to the ongoing debate about whether stylolites are conduits or barriers for fluid flow.

The architecture of basalt reservoirs in the North Atlantic Igneous Province with implications for basalt carbon sequestration

Abstract Offshore CO 2 sequestration in basaltic formations of the North Atlantic Igneous Province may allow permanent storage of large volumes of CO 2 through rapid carbonate mineralization. Characterizing the internal architecture of such reservoirs is key to assessing the storage potential. In this study, six photogrammetry models and three boreholes on the Faroe Islands have been used to characterize the internal lava sequence architectures as a direct analogue to potential offshore North Atlantic Igneous Province storage sites. The studied formations are dominated by c. 5 to 50 m thick simple and compound lava flows, with drill core observations documenting a transition from pāhoehoe moving towards ‘a’ā lava flow types interbedded with thin (<5 m thick) volcaniclastic rock units. The identification of flow margin breccias is potentially important as these units form excellent reservoirs in several other localities globally. Stacked, thick simple flows may present sealing units associated with dense flow interiors. Connected porous and permeable lava flow crusts present potential reservoirs; however, the degree of secondary mineralization and alteration can alter initially good reservoir units to impermeable barriers for fluid flow. Large-scale reservoir volumes may be present mainly within both vesicular, fractured pāhoehoe and brecciated flow margins of transitional simple lava flows.

Ultrastructure of precapillary sphincters and the neurovascular unit.

Neurons communicate with vasculature to regulate blood flow in the brain, a process maintained by the neurovascular unit (NVU). This interaction, termed neurovascular coupling, is believed to involve astrocytes or molecules capable of traversing the astrocytic endfeet. The precise mechanism, however, remains elusive. Using large 3D electron microscopy datasets, we can now study the entire NVU in context of vascular hierarchy. This study presents evidence supporting the role of precapillary sphincters as a nexus for neurovascular coupling and endothelial transcytosis. It also highlights the role of fibroblast-synthesized collagen in fortifying first-order capillaries. Furthermore, I demonstrate how astrocytic endfeet establish a barrier for fluid flow and reveal that the cortex's microvasculature is semicircled by an unexpected arrangement of parenchymal neuronal processes around penetrating arterioles and arterial-end capillaries in both mouse and human brains. These discoveries offer insights into the NVU's structure and its operational mechanisms, potentially aiding researchers in devising new strategies for preserving cognitive function and promoting healthy aging.

High-resolution sequence stratigraphy applied to turbidites: The case study of jurassic los molles formation, Neuquén basin, Argentina

High-Resolution Sequence Stratigraphy is fundamental for attaining a refined and representative geological 3D model for oil and gas reservoirs. For its application on turbidite systems it is essential to have a robust facies and facies association framework, recognizing architectural elements and the cyclicity in different frequencies on the stacking pattern. This study is widely based on a multi-scale approach (4th and 5th orders), integrating satellite image, outcrop descriptions and microscopic data to achieve a predictive depositional model through time. The Jurassic Los Molles Formation, Neuquén Basin, was chosen as a case study due to its outcrop quality of offshore shelf (Carreri section) and slope/basin plain settings (La Jardinera section). This enabled a rich comparison between facies associations, architectural elements, stacking patterns and cyclicity on turbidite systems of different palaeo physiographic settings. The Carreri offshore shelf turbidites display higher confinement with predominance of tractive structures and coarser grain-sizes as opposed to deepwater deposits. Their base level variations are recorded by the stacking of turbidite channel-overbank systems to delta front deposits and tidal bars that culminate important regressions. In contrast, in La Jardinera slope and basin plain succession, the falling stage system tract of turbidite sequences is characterized by amalgamated composed lobes and wide composed channels with coarser grain-sizes and erosive contacts. The lowstand system tract is represented by less amalgamated lobes and interlobes, suggesting the system backstepping until the muddy transgressive system tract. The workflow of this study allows a better understanding of the evolution of a sedimentary system, correlating this to important stratigraphic surfaces that can represent fluid flow barriers and reservoir heterogeneities that could impact on production data. High resolution sequence stratigraphy, especially in turbidite systems, is key for more accurate reserve estimates and for developing predictive and economic exploration and production strategies in the petroleum industry.

2D modeling and simulation of deformation bands’ effect on fluid flow: Implications for hydraulic properties in siliciclastic reservoirs

Deformation bands can influence fluid flow within reservoirs, mainly acting as flow barriers. This study aims to characterize deformation bands and evaluate the influence of these structures on fluid flow through 2D modeling and numerical simulation using the equivalent permeability tensor computed numerically through a flow-based upscaling procedure in siliciclastic reservoirs. To do so, we analyze arkosic sandstones of the Antenor Navarro Formation in the Rio do Peixe Basin, NE Brazil. The characterization of deformation bands involved: 1) acquisition of aerial imagery using a drone; 2) conducting direct field-based structural analysis along one scanline (frequency of deformation bands/m); and 3) collection of samples in the field to perform petrophysical (porosity and permeability) tests. After the characterization phase, we built a 2D geological model using the previously acquired parameters and performed a single-phase numerical flow simulation of water using finite element code. We simulated four different horizontal and vertical flow scenarios for both measured and exaggerated host rock permeability. Our structural analysis demonstrated that the deformation bands preferentially strike NE–SW, with secondary E–W– to N–S–orientations. Our petrophysical analysis showed that samples affected by deformation bands have lower porosity (by up to two orders of magnitude) and permeability (by up to four orders of magnitude) values than samples collected in the host rocks. The numerical fluid flow simulation allowed us to conclude that the deformation bands act as partial flow barriers where the hydraulic head can indicate the barrier effect intensity. Pressure drops between host rock regions separated by deformation bands reached values from 10 to 40%, depending on the fluid flow direction, especially for cases with a high contrast between the permeability values (three orders of magnitude) of the deformation bands and the host rock. The results of this study focus on the importance of contrasts in permeability measurements between deformation bands and host rocks, which could help to enhance understanding of the behavior of the deformation bands as partial fluid flow barriers, as well as their implications for hydraulic properties in deformed siliciclastic reservoirs.

High-resolution sequence stratigraphy applied for the improvement of hydrocarbon production and reserves: A case study in Cretaceous fluvial deposits of the Potiguar basin, northeast Brazil

High-resolution sequence stratigraphy (HRSS) was applied to Cretaceous fluvial deposits of Açu Formation, the principal oil-bearing unit of the Potiguar Basin, aiming to help reservoir management. Facies associations and stratigraphic surfaces were interpreted from an integrated analysis based on rock cores, ichnological, well logs, and production data. Overbank strata envelop transgressive meandering fluvial deposits and promote vertical fluid-flow barriers between reservoirs, thus characterizing zone boundaries. Zone boundaries consist of high-frequency subaerial unconformities placed at the top of paleosol levels correlated throughout the study area. The correlation allowed identifying nine high-frequency sequences and established the reservoir zonation that replaced the former four lithostratigraphic zones. The chronostratigraphic framework supports a more detailed and predictive 3D numerical geological model that is much more representative of the meandering fluvial system and its heterogeneities. Furthermore, the high-resolution stratigraphic reservoir zonation helped revitalize production by identifying intervals with high water cut, which hinder oil production, and oil-saturated intervals that were never opened to production. This case study demonstrates that the application of HRSS methodology is fundamental to improve oil reserves, production, and the final recovery factor in the Potiguar Basin oil fields and analogous situation worldwide.

Integration of geophysical and geomechanical data to understand the depletion of the Marlim field, Campos Basin

Located in the Campos Basin, Brazil, the Marlim field, consists of two turbidite systems deposited during eustatic sea-level variations in the Oligocene/Miocene. The reservoir was discovered in 1985, and its production started to decline in 2002. One of the techniques selected to assist in the recovery of oil from the reservoir was the 4D seismic. However, its interpretation can be complex. In order to help address this issue, the present study proposed an analysis of the depletion of a small field area from 1997 to 2010, combining geophysical (4D seismic) and geomechanical (pore pressure) data through the construction of pore pressure 3D models for both years, which can be subtracted and compared to seismic anomalies. The results obtained were: an average depletion of 0.42 ppg (50.33 kg/m3) of pore pressure gradient in the field; the identification of potential fluid-flow barriers, such as an NW-SE-oriented channel and sealing faults; and the detection of two areas with an expressive presence of 4D seismic anomalies, one of them showing a quite evident difference between pore pressure gradients, suggesting field depletion. The use of very old and noisy seismic data hindered the application of this methodology. Nevertheless, this research demonstrated the relevance of estimating pore pressure in the reservoir and how this geomechanical parameter can be useful in assessing the level of field depletion.

Distribution, microphysical properties, and tectonic controls of deformation bands in the Miocene subduction wedge (Whakataki Formation) of the Hikurangi subduction zone

Abstract. We analyse deformation bands related to horizontal contraction with an intermittent period of horizontal extension in Miocene turbidites of the Whakataki Formation south of Castlepoint, Wairarapa, North Island, New Zealand. In the Whakataki Formation, three sets of cataclastic deformation bands are identified: (1) normal-sense compactional shear bands (CSBs), (2) reverse-sense CSBs, and (3) reverse-sense shear-enhanced compaction bands (SECBs). During extension, CSBs are associated with normal faults. When propagating through clay-rich interbeds, extensional bands are characterised by clay smear and grain size reduction. During contraction, sandstone-dominated sequences host SECBs, and rare CSBs, that are generally distributed in pervasive patterns. A quantitative spacing analysis shows that most outcrops are characterised by mixed spatial distributions of deformation bands, interpreted as a consequence of overprint due to progressive deformation or distinct multiple generations of deformation bands from different deformation phases. As many deformation bands are parallel to adjacent juvenile normal faults and reverse faults, bands are likely precursors to faults. With progressive deformation, the linkage of distributed deformation bands across sedimentary beds occurs to form through-going faults. During this process, bands associated with the wall-, tip-, and interaction-damage zones overprint earlier distributions resulting in complex spatial patterns. Regularly spaced bands are pervasively distributed when far away from faults. Microstructural analysis shows that all deformation bands form by inelastic pore collapse and grain crushing with an absolute reduction in porosity relative to the host rock between 5 % and 14 %. Hence, deformation bands likely act as fluid flow barriers. Faults and their associated damage zones exhibit a spacing of 9 m on the scale of 10 km and are more commonly observed in areas characterised by higher mudstone-to-sandstone ratios. As a result, extensive clay smear is common in these faults, enhancing the sealing capacity of faults. Therefore, the formation of deformation bands and faults leads to progressive flow compartmentalisation from the scale of 9 m down to about 10 cm – the typical spacing of distributed, regularly spaced deformation bands.

Occurrence, mineralogy and geochemistry of fracture fillings in tight sandstone and their constraints on multiple-stage diagenetic fluids and reservoir quality: An example from the Kuqa foreland thrust belt, Tarim Basin, China

Sandstone reservoirs with a depth of over 6000m of the Lower Cretaceous Bashijiqike Formation in the Kuqa foreland thrust belt, Tarim Basin in NW China provide a good example for studying multiple-stage diagenetic fluid activities and effects of fractures on reservoir quality in tight sandstone. Three types of fractures can be identified in the sandstone: fully sealed fractures, partly filled fractures and minimally filled fractures. Fracture fillings are mainly composed of ferruginous dolomite, dolomite, calcite, quartz and gypsum. The ferruginous dolomite is mainly accumulated in the fully sealed fractures. The dolomite primarily occurs in the partly filled fractures and fully sealed fractures. The calcite and gypsum frequently occur in the fully sealed fractures, partly filled fractures and minimally filled fractures. The quartz is distributed at inner edges of the partly filled fractures. Contents of Co, Ni and V in the ferruginous dolomite are generally higher than that in the dolomite, calcite and gypsum, and contents of REE and LREE gradually decrease in turn from the ferruginous dolomite to the dolomite, and to the calcite. The δ13C of the ferruginous dolomite, dolomite and calcite ranges from 3.78‰ to −1.49‰ with an average value of −2.82‰, and their δ18O ranges from −16.43‰ to −12.3‰ with an average value of −14.62‰, respectively. The fully sealed fractures, ferruginous dolomite, partly filled fractures, quartz, dolomite, minimally filled fractures, calcite and gypsum successively occur from early to late stage. Data of δ13C and δ18O of the carbonate minerals in the fractures indicate that these minerals are formed in freshwater which is oxidizing with low salinities, and average formation temperature of the ferruginous dolomite, dolomite, and calcite is 89 °C, 109 °C and 123 °C, respectively. Formation of these carbonate minerals in the fractures is primarily related to hydrothermal fluids originated from pyrolysis of organic matter. The fully sealed fractures serve as a barrier for fluid flow whereas the partly filled fractures and minimally filled fractures both provide channels for hydrocarbon migration and accumulation.

The North Pyrenean Frontal Thrust: structure, timing and late fluid circulation inferred from seismic and thermal-geochemical analyses of well data

During orogenesis, large-scale thrusts as orogenic fronts can act as conduits and/or barriers for fluid flow. Unravelling the timing and modes of tectonic activation of large-scale faults is crucial to understanding the relationship between fluid flow and deformation. The North Pyrenean Frontal Thrust (NPFT) corresponds to a major basement-involved thrust responsible for the northward overthrust of the pre-orogenic sediments on top of the Aquitaine Foreland Basin. This study questions the timing of activation of this thrust, its geometry, the nature of the last fluids, which circulated there, and its role on the circulation of fluids. The structural study confronted to published thermochronology data led to determine the timing of the two tectonic activations during the NPFT compression phase and to relate them to the fluid circulations. We constrain the first activation at Campanian times and link it to the leak of the deep gas reservoir present in depth, as the NPFT acted as a conduit. Then the NPFT acted as a barrier, probably due to the breccia consolidation during the Paleocene quiescence period. Finally, the Eocene-Oligocene reactivation led to fluid circulation of high salinity fluids from the Triassic evaporites leaching. This latter event is associated with a fracturing event and the late generation of calcite veins studied here. This is the first study in the Pyrenees directly applied to the NPFT which uses the association between fluid inclusions study, seismic and thermochronological data. It highlights that the NPFT may be an important structure responsible of the leakage of deep hydrocarbons reservoirs. It also shows the importance of the determination of the activation steps of large-scale faults to decipher the origin of fluid circulations in space and time.

Petrofacies of Eocene sand injectites of the Tumey Giant Injection Complex, California (USA)

The forearc succession of Great Valley Group in Central California provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface. Several sand injection complexes are well described in the outcrop and subsurface, however the petrographic characteristics of injectites are still poorly documented. In this paper, we present the results of an integrated study of field observations, quantitative and qualitative sandstone petrography, provenance, and petrofacies analysis of the Tumey Giant Injection Complex (TGIC) in order to understand its lithostratigraphy and petrological evolution, and its impacts on reservoir petrofacies characteristics and fluid migration. The TGIC intrudes into a 450 m thick deep-water succession of slope mudrocks and sandy channel-fills of the Kreyenhagen Shale (Eocene), forming an interconnected network of sandstone sills, dykes and injection breccias. The complex generated a horizontal and vertical plumbing system for fluid migration, connecting isolated sandy channel-fills among low-permeability mudrocks. The primary detrital composition, diagenetic products, microtextures, and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations. Petrofacies associations confirm that the gypsum-cemented feldspathic litharenites from the Kreyenhagen Shale channel-fills are the only source for the injection complex. Eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex, while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity. The underlying calcite-cemented arkosic sandstones of the Lodo and Domengine formations acted as barriers for fluid flow, aiding lateral fluid migration and overpressure buildup within the overlying Kreyenhagen channel-fills. Intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks. This study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere.

Structural Control of a Dissolution Network in a Limestone Reservoir Forced by Radial Injection of CO2 Saturated Solution: Experimental Results Coupled with X-ray Computed Tomography

This study was conducted in the framework of the PILOT CO2-DISSOLVED project, which provides an additional approach for CO2 sequestration, with the aims of capturing, injecting, and locally storing the CO2 after being dissolved in brine. The brine acidity is expected to induce chemical reactions with the mineral phase of the host reservoir. A set of continuous radial CO2 flow experiments was performed on cylindrical carbonate rock samples under geological storage conditions. The objective was to interpret the dissolution network morphology and orientation involved. To explore the three-dimensional architecture of dissolution arrays and their connection integrity within core samples, we used computed tomography. A structural investigation at different scales revealed the impact of the rock heterogeneity on the dissolution pathways. The initial strike of the observed mesoscopic wormholes appears to be parallel to dilatational fractures, with a subsequent change in major trends of dissolution along master shears or, more specifically, a combination of synthetic shears and secondary synthetic shears. Antithetic shears organize themselves as slickolitic surfaces, which may be fluid-flow barriers due to different mineralogy, thus affecting the permeability distribution-wormhole growth geometry induced by CO2-rich solutions.

Fluid flow properties of the Wilhelmøya Subgroup, a potential unconventional CO2 storage unit in central Spitsbergen

The Upper Triassic to Middle Jurassic Wilhelmoya Subgroup forms one of the more suitable reservoir units on the Norwegian Arctic archipelago of Svalbard. The target siliciclastic storage unit, which is encountered at approx. 670 m depth at the potential injection site in Adventdalen, central Spitsbergen, is a severely under-pressured (at least 35 bar), tight and compartmentalised reservoir with significant contribution of natural fractures to permeability. In this contribution, we characterise the 15–24 m-thick Wilhelmoya Subgroup storage unit using both borehole and outcrop data and present water-injection test results that indicate the presence of fluid-flow barriers and the generation of new, and propagation of pre-existing natural fractures during injection. Whole core samples from drillcores and outcrops were sampled for pore network characterisation and analysed using high-resolution X-ray computed tomography (Micro-CT). We demonstrate that heterogeneities such as structural discontinuities, igneous bodies and lateral facies variations, as examined in well core and equivalent outcrops, will strongly influence fluid flow in the target reservoir, both by steering and baffling fluid migration. Many of these heterogeneities are considered to be subseismic, and their detailed characterisation is important to predict subsurface CO2 storage potential and optimise injection strategy.

Across‐axis variations in petrophysical properties of the North Porcupine Basin, offshore Ireland: New insights from long‐streamer traveltime tomography

AbstractThe Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (Vp) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D Vp structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new Vp–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The Vp model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The Vp model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.

Stratigraphic Controls on Petrophysical Attributes and Fluid-Flow Pathways in an Exhumed Fluvial Reservoir

Fluvial, floodplain and lake strata of the Green River Formation (Eocene) occur within an exhumed oil reservoir exposed in a quarry near Sunnyside, Utah. Strata in the quarry highwalls define three, asymmetrical, 15- to 20-m thick, base-level-rise genetic sequences arranged in a long-term base-level-rise (landward stepping) stacking pattern. Variable intensity of oil stain on rock surfaces is a qualitative measure of pore volumes, as all permeable facies are fully saturated with oil. Visual estimates of oil-stain intensity, combined with petrophysical measurements and petrographic analysis of the different facies, were used to define fluid-flow compartments and their boundaries. Strata and facies that functioned as fluid-flow conduits, retardants and barriers were mapped on photomosaics of the quarry highwall. Three separate fluid-flow compartments coincide with the three genetic sequences. Amalgamated fluvial sandstones at the base of each genetic sequence functioned as flow units of varying permeability and degree of interconnectedness. Laterally continuous floodplain and/or lacustrine mudstones, which cap each genetic sequence, entirely lack oil in matrix porosity and functioned as fluid-flow barriers and compartment boundaries. Petrophysical properties of specific sedimentary facies are sensitive to stratigraphic position at three spatial scales, even though the sedimentary facies appear nearly identical. At the long-term scale, porosity and permeability of the same facies (trough cross-stratified sandstone is the most common) in channel sandstones of the three genetic sequences decrease in stratigraphic succession. Within each genetic sequence, porosity and permeability are highest at the base and decrease quasilinearly to the top. Using oil-stain intensity as a proxy, porosity and permeability generally decrease from base to top of each scour-based channel macroform. Petrophysical variations coincide with subtle variations in grain size and trough cross-stratification set thickness within otherwise indistinguishable sedimentary facies. These results demonstrate that conventional crossplots of porosity/permeability versus sedimentary facies are unnecessarily broad and imprecise. When such petrophysical data are plotted in a stratigraphic context, porosity and permeability values have significantly reduced scatter. Porosity and permeability measurements and predictions of each sedimentary facies should be made from a stratigraphic perspective. From our observations of variations in intensity of oil stain, homogeneity of fluid flow may not be equated directly with facies homogeneity. At one extreme of an apparent continuum, fluid-flow pathways are tortuous and extremely variable within homogeneous, high permeability, amalgamated channel sandstones. Sweep efficiencies may be low in these cases. At an intermediate position in the continuum, increased diversity of sedimentary facies and stratigraphic variability usually cause sufficient stratigraphic separation of permeable and impermeable strata such that fluid-flow pathways are more confined and have a reduced tortuosity. Sweep efficiencies may be high in these cases. At the other extreme of the continuum, where diversity of sedimentary facies and stratigraphic variability is very high, stratigraphic units are discontinuous and restricted in area. In such cases, fluid-flow pathways are not laterally connected, and sweep efficiencies would be low.

Origin of carbonate cements with implications for petroleum reservoir in Eocene sandstones, northern Dongying depression, Bohai Bay basin, China

The Eocene Es4s interval is an important petroleum reservoir of sublacustrine fan dominated at the depths of 2500–4000 m in Dongying depression, Bohai Bay basin. Based on core observation, three types of carbonate-cemented beds have been identified and commonly contain ferroan calcite and ankerite predominantly and less calcite and dolomite. Precipitation temperatures range from 34.6 to 72.8℃ for calcite and dolomite cements, and from 110 to 153℃ for ferroan calcite and ankerite cements. The high δ13CPDB values (−0.65 to +5.59‰) for calcite and dolomite suggest that dissolved inorganic carbon, derived from methanogenic fermentation of organic matter in adjacent mudstones. The low δ13CPDB values (+1.04 to +3.29‰) for ferroan calcite and ankerite probably indicate a mixture of carbon derived from decarboxylation of organic acid as well as from the dissolution of early formed carbonate cements. High plug porosity is mainly developed at the central section of sandstones vertically and the porosity decreases sharply toward the top and base of the sandstones due to extensively carbonate-cemented beds. The carbonate-cemented beds varies from 0.02 to 0.5 m in thickness and might extend from tens to hundreds meters laterally. It could be served as fluid-flow barriers and seals for petroleum, and result in reservoir deterioration and significant heterogeneity.

Diagenesis and Diagenetic Evolution of Deltaic and Neritic Gas‐Bearing Sandstones in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin: Implications for Depositional Environments and Sequence Stratigraphy Controls

AbstractGas‐bearing deposits in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin consist of shoreface sandstones of the highstand systems tract (HST) and transgressive systems tract (TST), and deltaic sandstones of the lowstand systems tract (LST) and falling stage systems tract (FSST). Detailed petrographic observations suggest that the diagenetic features and related evolution of these deposits cannot be simply characterized and demonstrated in the depth domain. However, the occurrence of diagenetic minerals systematically depends on the studied interval within the HST, TST, LST, and FSST; therefore, diagenesis in this region can be better constrained when studied in the context of the depositional environments and sequence stratigraphic framework. The eogenetic processes in such settings include: (1) microcrystalline siderite precipitated as concretions in almost all environments and systems tracts, which inhibited further mechanical compaction; (2) grain dissolution and kaolinitization occurred in shoreface HST sandstones and deltaic LST and FSST sandstones; (3) glaucony was locally observed, which did not clearly reflect the controls of facies or sequence stratigraphy; and (4) cementation by pyrite aggregates occurred in the shoreface HST sandstones and deltaic LST sandstones. The mesogenetic diagenesis includes: (1) partial conversion of kaolinite into dickite in deltaic LST sandstones, and minor chlorite cementation in deltaic FSST sandstones; (2) transformation of kaolinite into illite and quartz cementation in deltaic LST and FSST sandstones; (3) frequent precipitation of ankerite and ferroan calcite in shoreface TST sandstones and early HST sandstones, forming baffles and barriers for fluid flow, with common calcite in shoreface HST sandstones as a late diagenetic cement; and (4) formation of dawsonite in the deltaic LST and FSST sandstones, which is interpreted to be a product of the invasion of a CO2‐rich fluid, and acts as a good indicator of CO2‐bearing reservoirs. This study has thus constructed a reliable conceptual model to describe the spatial and temporal distribution of diagenetic alterations. The results may provide an entirely new conceptual framework and methodology for successful gas exploration in the continental margins of offshore China, thus allowing us to predict and unravel the distribution and quality evolution of clastic reservoirs at a more detailed and reliable scale.

Effects of carrier bed heterogeneity on hydrocarbon migration

Reservoir/carrier bed heterogeneity is a common geological phenomenon. This study applied a hydrocarbon migration model based on the invasion percolation theory to simulate the formation of hydrocarbon migration pathways in heterogeneous carrier beds. The results indicate that the pathways and hydrocarbon accumulations in reservoirs may differ from conventional models when continuous low-permeability layers occur as networks within reservoirs. Such low-permeability layers may have formed through depositional or diagenetic processes within reservoir rocks at variable scales. Oil-bearing reservoir in Donghe Sandstone in Hadexun Oilfield in northern Tarim Basin, China, was analyzed to demonstrate the forming mechanisms of substantial tilting of oil-water contact. Donghe Sandstone is a quartz sandstone deposited in an open marine shoreface-shelf environment during a major transgression in Early Carboniferous. . Major diagenetic products are abundant tabular concretions along bedding planes and cross laminae. The low-permeability concretions serve as fluid flow barriers which compartmentalize the Donghe Sandstone reservoir. As a result, the oil-water contacts and the saturation of oil in the reservoir appear irregular and tilted in Hadexun Oilfield and neighboring regions.

Accurate control of hyperbolic trajectories in any dimension.

The unsteady (nonautonomous) analog of a hyperbolic fixed point is a hyperbolic trajectory, whose importance is underscored by its attached stable and unstable manifolds, which have relevance in fluid flow barriers, chaotic basin boundaries, and the long-term behavior of the system. We develop a method for obtaining the unsteady control velocity which forces a hyperbolic trajectory to follow a user-prescribed variation with time. Our method is applicable in any dimension, and accuracy to any order is achievable. We demonstrate and validate our method by (1) controlling the fixed point at the origin of the Lorenz system, for example, obtaining a user-defined nonautonomous attractor, and (2) the saddle points in a droplet flow, using localized control which generates global transport.

Hydrothermal alteration in an exhumed crustal fault zone: Testing geochemical mobility in the Caleta Coloso Fault, Atacama Fault System, Northern Chile

Crustal scale strike slip fault zones have complex and heterogeneous permeability structures, playing an important role in fluid migration in the crust. Exhumed faults provide insights into the interplay among deformation mechanisms, fluid–rock interactions and bulk chemical redistributions. We determined the whole-rock geochemistry and mineral chemistry of the fault core of the Caleta Coloso Fault in Northern Chile, in order to constrain the physical and chemical conditions that lead to strong hydrothermal alteration. The strike-slip Caleta Coloso Fault core has a multiple-core architecture, consisting of alternate low strain rocks (protolith, weakly deformed protolith and protocataclasites) and high-strain strands (cataclasites and discrete band of ultracataclasite) derived from a Jurassic tonalite. Hydrothermal alteration associated with fault-related fluid flow is characterized by a very low-grade association consisting of chlorite, epidote, albite, quartz and calcite. Chlorite thermometry indicates T-values in the range of 284 to 352 °C, no variations in mineral composition or T-values were observed among different cataclastic units. Mass balance and volume change calculations show significantly larger chemical mobility in the protocataclasites than in the cataclasite (and ultracataclasite). This suggests that fluid flow and chemical alteration are strongly controlled by deformation being protocataclasite relatively more permeable than cataclasite. Chlorite precipitation and grain reduction in cataclasite (and ultracataclasite) would reduce permeability acting as a barrier for fluid flow. Chemical mobility and volume changes in the Coloso Fault core suggest different effective fluid/flow ratios during amalgamation of subsequent and subparallel deformation bands that finally control the fracture–channeling allowing fault-related fluid–flow into each of them. • We examine internal architecture of an ancient brittle fault zone in Northern Chile. • Fault zone consists of a multiple core of high/low strain cataclastic rocks. • Fault-related fluid flow is characterized by a very low-grade mineral association. • Chemical mass balance and volume change calculations were documented. • Major chemical mobility occurred in the highest permeable rocks.