New and interesting

This section highlights a selection of recently published, interesting scientific articles dealing with evaporite-related topics, along with a link that takes you to the article source.

Depositional model of early Cretaceous lacustrine carbonate reservoirs of the Coqueiros formation - Northern Campos Basin, southeastern Brazil

Recent discoveries of giant hydrocarbon accumulations in coquinas and other lacustrine carbonate rocks of Rift and Post-rift (pre-salt) super-sequences of the Campos and Santos Basins have motivated a series of studies focused on understanding the genesis of these unique reservoirs. An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Coqueiros Formation delineated the depositional model and evolution of reworked bioclastic rocks and other associated rocks deposited on the external high in the northern Campos Basin during the transition between late-rift and sag stages. Bioclastic rudstones and grainstones with moderate to high shell reworking represent 65% of the samples described. The following pieces of evidence point to a predominantly shallow lacustrine depositional environment, dominated by storm currents: (i) presence of high-angle (>14°) longshore cross-bedded rudstones; (ii) limited occurrence of muddy sediments interbedded with thick packages of amalgamated bioclastic rudstones; (iii) great lateral continuity of rudstone deposits devoid of matrix on the structural high, consistent with processes of storm action on large expanses of shallow lakes; (iv) deposits of massive rudstones, with disorganized shells, similar to the modern coastal bioclastic deposits of California and Shark Bay. Three sets of facies association are proposed: (i) high-energy facies, composed of bioclastic rudstones and grainstones devoid of matrix deposited above the fair weather wave base (FWB); (ii) low energy facies, composed of hybrid rocks with muddy or peloidal matrix and other fine grained rocks deposited below the FWB or in protected shallow environments; (iii) an alkaline set, composed of mainly magnesian clay ooids deposited in a more alkaline composition of lake water. The evolution of the facies associations over time demonstrates an overall fall of the relative lake level. High proportion of low energy facies associated with a deeper lacustrine environment occurs at the Coqueiros Fm. base, while at the top predominate high energy facies associated with a shallow lacustrine environment, in addition to magnesian clay-rich facies associated to restricted and alkaline water bodies. Paleocurrents suggest the activity of two main wind direction (northwest and southeast) over the external high that result in clean bioclastic facies with good reservoir potential on both flanks. The knowledge of wind directions during the late Barremian and early Aptian in the northern Campos Basin is extremely important for targeting the best depositional facies for hydrocarbon accumulation in the Coqueiros Formation.

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Characterization of Oligo-Miocene evaporite-rich minibasins in the Sivas Basin, Turkey

The Sivas Basin in Turkey displays in its central part an Oligo-Miocene halokinetic province which acts as a major outcrop analogue to study salt-sediment interactions. Based on field geology observations, the paper focuses on the geometry and sedimentology of several minibasins having the particularity of being mainly filled by gypsiferous deposits. Such a type of evaporite-rich minibasins remain difficult to identify and so are poorly studied in other halokinetic provinces. In the Sivas Basin, the evaporites were recycled from diapiric salts and precipitated in saline ponds emplaced above deflating diapiric stems. Diapir deflation resulted either from local transtensive strain, cessation of diapir feeding and/or subsurface dissolution of the diapiric salt. Minibasin subsidence was likely enhanced by the fast emplacement rate of the capping evaporites, together with the high density of the depositional sulfates compared to the diapiric halite. The evaporite-rich minibasins stand out from their surrounding siliciclastic counter-parts by their small dimension (lower than 1 km-wide), their encased teardrop shape, and their high internal deformations. The later include well-developed halokinetic sedimentary wedges, aerial mega-slumps or inverted flaps. Such structural features probably resulted from the ductile rheology of the evaporite infill and the complex pattern of downbuilding. Although secondary evaporitic minibasins have never been identified in other ancient halokinetic settings, this study highlights that they could develop in any evaporitic environments, coastal or continental, such as in the Precaspian Basin. The secondary minibasins described here can also act as field analogues of other primary evaporite-rich minibasins already suspected in salt giant basins (e.g. in the Santos Basin, Brazil).

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Limited thermochemical sulfate reduction in hot, anhydritic, sour gas carbonate reservoirs: The Upper Jurassic Arab Formation, United Arab Emirates

Limited thermochemical sulfate reduction (TSR) in hot (130–160°C) and anhydrite-rich sour gas reservoir carbonates of the Arab Formation (Upper Jurassic) is manifested by rare calcitization of anhydrite with slightly lower δ13CVPDB values (−3.2 to −0.1‰) than calcite precipitated in equilibrium with Late Jurassic seawater. Fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that TSR occurred between 130°C and 160°C. The lack of evidence for extensive TSR, despite the suitable current temperatures and abundant sulfates in the gas reservoir, coupled with the relatively more common TSR-related calcite in the flanks (water zone) than crest (gas zone), indicate that: (1) gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited TSR in the crest even when it subsequently reached depths where extensive TSR would occur, and (2) H2S (up to 38 vol%) has migrated from the underlying Permo-Triassic and/or Jurassic sulfate-carbonate deposits. This study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial-thermal history is crucial for a better understanding of the origin and distribution of H2S in hot, anhydrite-rich, sour gas reservoirs.

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Schematic conceptual model of the extent of TSR inthe Arab Formation across the anticline of the field. (A)Formation of the anticline and gas migration occurred in LateCretaceous during the obduction of the Oman ophiolites. TSRstarted in the flanks at around 130 °C in Early Eocene duringgas migration when the reservoir temperatures were too lowto cause extensive TSR in the crest. (B) TSR continued in theflanks and commenced in the lower parts (i.e. outer ramp) ofthe crest but not in the upper parts (i.e. supratidal and lagoon)because of early gas emplacement in Early to LateEocene. (C) TSR continued in the water-saturated flanks,whereas stopped or slowed down significantly in the crestbecause of gas emplacement during the Late Eocene.

Hydrodynamics of salt flat basins: The Salar de Atacama example

The Salar de Atacama is one of the most well-known saline endorheic basins in the world. It accumulates the world main lithium reserves and contains very sensitive ecosystems. This paper characterize the hydrodynamics of the Salar de Atacama, and quantifies its complex water balance prior to the intense brine extraction. The methodology and results can be extrapolated to the groundwater flow and recharge of other salt flats. A three-dimensional groundwater flow model using low computational effort was calibrated against hundreds of hydraulic head measurements.

The water infiltrated from the mountains ascends as a vertical flux through the saline interface (mixing zone) produced by the density contrast between the recharged freshwater and the evaporated brine of the salt flat nucleus. This water discharges and is largely evaporated from lakes or directly from the shallow water table. On the other hand, the very low hydraulic gradients, coupled with the presence of the mixing zone that operates as barrier, leads the salt flat nucleus to act as a hydrodynamically quasi-isolated area. The computed water table shows the lowest hydraulic head in the salt flat nucleus near the discharge at the mixing zone. The groundwater balance of the Salar de Atacama in its natural regime was quantified resulting in an inflow/outflow of 14.9 m3·s−1. This balance considers the basin as an endorheic system. The very low infiltration values that are generally assumed for hyperarid basins are not consistent with the hydrogeology of the Salar de Atacama. Indeed, very high infiltration rates (up to 85% of rainfall) occur because of the high degree of fracturing of rocks and the scarce vegetation. This high infiltration is consistent with the light isotopic composition of the water from the recharge area (Altiplano). Therefore, the existence of additional inflows outside the basin is unlikely.

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Deposition, diagenetic and hydrothermal processes in the Aptian Pre-Salt lacustrine carbonate reservoirs of the northern Campos Basin, offshore Brazil

The discovery of large oil accumulations in the rift and sag Pre-Salt sections of the Campos and Santos Basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the Brazilian and African marginal basins. More than half of Brazilian oil production originates from the Pre-Salt reservoirs of these offshore basins. A study integrating systematic petrography, cathodoluminescence, scanning electron microscopy, microprobe and X-ray diffraction was performed on seven wells in the northern Campos Basin. This study highlights the major primary, diagenetic and hydrothermal features of the Pre-Salt succession, with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs. The Pre-Salt deposits correspond to bioclastic grainstones and rudstones, syngenetic crusts of fascicular calcite, and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites. Magnesian silicates are frequently associated with carbonate deposits. In the sag phase, stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites, which displace and replace the syngenetic magnesian clay deposits. In the rift section, stevensitic ooids are mixed with bioclasts or form ooidal arenites. Pre-Salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution. Eodiagenetic processes include the dissolution, neomorphism and cementation of bivalve bioclasts in the rift, as well as the dissolution of magnesian silicates and their replacement by calcite spherulites, silica and dolomite in the sag section. Burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems. These fluids promote dolomitization, silicification, and dissolution at varying degrees and intensities. Eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation, redistribution, and obliteration of porosity in the Pre-Salt reservoirs.

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Schematic representation of the genesis of typical Aptian Pre-salt deposits. A) Laminated deposits of syngenetic magnesian clays,with dispersed clay peloids and siliciclastic grains; B) Partial replacement and deformation of the Mg-clay matrix by calcite spherulites. Asymmetrical spherulites formed closer to the water-sediment interface (WSI); C) Non-coalesced fascicular aggregates of calcite precipitated on WSI with inter-aggregate growth-framework porosity. Clay peloids and siliciclastic grains included in some fascicular aggregates; D) Characteristic “cycle” showing the syngenetic crust of coalesced fascicular calcite aggregates at the top, and syngenetic Mg-clay matrix partially replaced and displaced by calcite spherulites in the middle and preserved at the base.
Origin of the Neoproterozoic rim dolomite as lateral carbonate caprock, Patawarta salt sheet, Flinders Ranges, South Australia

The “rim dolomite” of South Australia?s Central Flinders Ranges is a prominent ridge-forming, layered dolomitic and siliceous unit. The rim dolomite is interpreted to be a lateral caprock found exclusively at the salt?sediment interface between the Patawarta diapir and the Ediacaran-aged Bunyeroo Formation. Lateral dolomite caprock is defined by the following field relationships: (1) the rugose dolomicrite base that parallels the contact of the diapiric matrix and the bedding in the overlying stratigraphy, (2) the exclusive presence of dolomite at the salt?sediment interface, (3) the lack of sedimentary structures or fossils (cyanobacterial laminites and stromatolites), (4) the lack of interbedded Bunyeroo lithofacies, and (5) the inability to trace the rim dolomite capstone away from the diapir margin into the outboard stratigraphy. In addition to the field relationships, the rim dolomite displays the following capstone fabrics: (1) massive?microcrystalline dolomite, (2) porphyritic?two distinct crystal sizes, one forming microcrystalline dolomite groundmass and the other forming rosettes of silica, (3) banded?microcrystalline dolomite forming pressure-dissolution layers of silica and authigenic hematite, and (4) brecciated?mosaic to disorganised, forming a microcrystalline dolomite groundmass, which locally contains remnant clasts of Callanna non-evaporite lithologies, such as quartz arenite to arkosic sandstones and basalts, surrounded by an anastomosing cement-filled vein network. All capstone fabrics contain various amounts of anhydrite, quartz, feldspar and non-evaporite grains that represent the insoluble residue during halite dissolution and caprock accretion. Three different genetic models for the lateral caprock are described and tested, and that of these, only the halokinetically rotated caprock model fits the data. The field relationships and capstone fabrics of the rim dolomite match other lateral caprocks in salt basins such as the Paradox Basin and Gulf Coast, USA.

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Rim dolomite at the salt–sediment interface at the Patawarta diapir.
Models of lateral caprock formation, halokinetically rotated caprock; (1) Lower BunyerooFormation is deposited and covers the Patawarta diapir; (2) rim dolomite develops as a crestal caprock; (3) diapir inflation causes extension and fracturing ofroof panel and rim dolomite; (4) continued diapir inflation causes erosion of the roof and rim dolomite, clasts of which are subsequently deposited in the overlyingBunyeroo Formation; (5) the rim dolomite is subsequently buried by the Wonoka Formation forming a halokinetic sequence boundary. With further halokineticrotation , the rim dolomite becomes vertical to overturned in a lateral position.
Stylolite-controlled diagenesis of a mudstone carbonate reservoir: A case study from the Zechstein_2_Carbonate (Central European Basin, NW Germany)

Stylolites are rough dissolution surfaces that form due to intergranular pressure-solution resulting from burial compaction or tectonic stress. Despite being ubiquitous in most carbonate rocks, their potential impact on structural diagenesis and fluid flow remains unclear. The Zechstein 2 Carbonate (Ca2) is a diagenetically complex reservoir in the Southern Permian Basin and represents one of the most prolific gas reservoirs in NW Germany. This investigation focuses on evaluating the relationship between stylolites, fractures/veins and their subsequent influence on the spatial variations in reservoir quality. We utilise drill core samples to carry out a combined analysis of cross-cutting relationships between different structures and diagenetic products. We therefore use a combination of petrography and statistical analyses on stylolite networks, focusing on their occurrence, morphology and sealing capacity. In the study area, the Ca2 carbonate mudstone was deposited in a slope environment and dolomitised under shallow burial conditions, followed by bedding-parallel stylolitisation during burial. Results indicate that calcium-rich fluids percolated from neighbouring evaporite units causing widespread calcitisation within the more distal environments of deposition. Some stylolites locally acted as barriers to affect the migration of the calcitising fluids, resulting in a macroscopic diagenetic stratification of relatively porous dolomite and areas of calcitised dolomite with lower porosity. However, pressure-solution continued during burial and bedding-parallel stylolites also appear postdating calcitisation. During inversion, horizontal stylolites were reopened to act as conduits to enable fluid migration that precipitated metal sulphides. This indicates that stylolites acted as both barriers and conduits for fluid flow depending on variations of the overburden pressure and regional stress regime. Stylolites present a range of sealing capacities between 63 and 89%, depending on their morphology, and can result in partial leakage and subsequent invasive calcitisation in their vicinity. This study highlights the importance of understanding the impact of stylolites on structural diagenesis and spatial variations in petrophysical rock properties that determine reservoir quality.

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Block diagrams indicating the changing role of stylolites observed inthe core. A. Stylolites are created in a dolomite matrix (dark grey) prior to calcitisation.B. Stylolites act as barriers to partially inhibit the vertical migration of calcitising fluid to create horizontal calcitised dolomite layers and patchy calcitised dolomite around stylolites (light grey). C. Stylolites are reopened during inversion, postdating calcitisation therefore not acting as a conduit for calcitised dolomite. D. Reopened stylolites act as conduits for iron-rich fluids which precipitate metal sulphides such as pyrite and galena (gold).

Antecedent aeolian dune topographic control on carbonate and evaporite facies: Middle Jurassic Todilto Member, Wanakah Formation, Ghost Ranch, New Mexico, USA

The Middle Jurassic Todilto Member of the Wanakah Formation is a carbonate and gypsum unit inset into the underlying aeolian Entrada Sandstone in the San Juan Basin. Field and thin section study of the uppermost Entrada and Todilto at Ghost Ranch, New Mexico, identified Todilto facies and their relationship to remnant Entrada dune topography. Results support the previous interpretation that the Entrada dunes, housed in a basin below sea level, were rapidly flooded by marine waters. Mass wasting of the dunes gave rise to sediment-gravity flows that mostly buried remnant dune topography, leaving ca 12 m of relief that defined the antecedent condition for Todilto deposition.

Previously interpreted as seasonal varves deposited in a stratified water body, the Todilto is reinterpreted as a microbial biolaminite. Most diagnostic are organic-rich laminae with structures characteristic of filamentous microbes and containing trapped aeolian silt, and clotted-texture laminae with a fabric associated with calcification of extracellular polymeric substances. The dune palaeotopography controls the spatial arrangement of Todilto facies. A continuous basal laminated mudstone thickens over the dune crest, reflecting the optimum conditions for microbial mat development, and is interpreted to have been deposited when marine waters submerged the topography.

Subsequent drying caused the emergence of the crestal area, the formation of tepee structures and a dissolution breccia. Gypsiferous mudflats and periodic ponds occupied the dune flanks and interdune area, with gypsum concentrated within the interdune area. Entrada sands remained unstable during Todilto deposition with common injection structures into the Todilto, and a remnant slope caused the downslope movement and folding of Todilto strata on the upper lee face. Although some expansion of the gypsum occurred in the subsurface, facies architecture fostered the development of a dissolution front adjacent to the interdune gypsum body with section collapse of gypsiferous limestone on the dune flanks.

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A microbial community developed on the submerged, remnant palaeotopography (facies T1, T2 and T5) during Stage 3. Falling water level gave rise to brines and initial precipitation of gypsum in the interdune depression (facies T2). By Stage 4 the crestal area was emergent and tepees formed (facies T1). Continued exposure of the crest gave rise to development of a dissolution breccia (facies T5). Downslope of the emergent crestal area, microbial mats formed on sabkha mudflats with gypsum nodules (facies T3) and gypsum (facies T6) was deposited in the ponded interdune area. During Stage 5 ponding events below the emergent crest occurred, with a hypothesized repetitive deposition of microbial mat, carbonate and gypsum laminations (facies T4). Gypsum (facies T7) was again concentrated within the interdune area. Stage 6 depicts subsurface collapse of facies T3 and, to a much greater extent, facies T4 along a dissolution front developed adjacent to the gypsum body.