Andrea Ceriani

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypts Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

Composition of modern stream sands derived from sedimentary source rocks in a temperate climate (Northern Apennines, Italy)

Abstract The Northern Apennine is a moderate relief thrust–fold belt (maximum elevation of less than 2000 m above sea level) lying in an area of temperate climate, drained by short (mean 50 km) transversal streams flowing northwards into the Po River main channel. The mountain range is mostly made-up of sedimentary rocks with minor ophiolite slices. The source rock types of nine contiguous watersheds, draining an overall area of more than 2800 km 2 , were quantitatively defined through a Geographical Information System (GIS). The bedrock lithology at the source area was compared with the composition of the medium-sized fluvial sand to evaluate how the various types of sedimentary source rocks are represented in the stream sand population. The sand provenance approach has two approximation levels; the first level distinguishes the source rock types and constrains some assumptions about the degree of recycling; the second level calculates the contribution of different source rock types according to the previous results. Provenance modelling obtained through this method fits well with the real geology of the eroded landscape. This provides an actualistic example that can be useful for a quantitative provenance approach to the study of ancient sediments fed by recycled sedimentary thrust–fold belts.

Integrated analyses constraining the provenance of sandstones, a case study: the Section Peak Formation (Beacon Supergroup, Antarctica)

Sandstones of the Section Peak Formation (late Triassic?‐Middle Jurassic), Northern Victoria Land, Antarctica, record a change of palaeotectonic regime, from a stable continental basement setting to the beginning of rifting indicated by widespread basaltic volcanism during the Middle Jurassic. The provenance study of sandstones was performed by integrating conventional microscope study of detrital mode, a varietal study of quartz grains, the detection of mineral inclusions in detrital quartz, and electron microprobe analyses of detrital garnets. This provides the basis for discussing the validity and limits of each of these approaches, highlighting the potential source of pitfalls in using a single provenance approach, especially when clastics related to rapidly changing geotectonic regimes are studied. The resulting palaeogeographic picture is of a braided stream system draining a low relief, crystalline basement area, composed mainly of granitoid and high-grade metamorphic rocks, intermittently interacting with coeval basaltic lava flows that were partially reworked by fluvial currents.

Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates

Abstract Microporosity in carbonate reservoirs is generated by the complex interplay between depositional and diagenetic processes. This petrographical, SEM, fluid-inclusion and isotopic study of a Lower Cretaceous carbonate reservoir, Abu Dhabi, UAE, revealed that: (1) micritization of ooids and skeletal fragments, which resulted in spheroidal (rounded) micrite, accounts for most microporosity in peloidal packstones and grainstones; and (2) transformation of spheroidal micrite into subhedral/euhedral micrite and microspar, known as aggrading neomorphism, could happen via precipitation of syntaxial calcite overgrowths around micrite (micro-overgrowths) and not only, as suggested previously in the literature, by recrystallization involving the dissolution (of micrite) and reprecipitation (of microspar). Precipitation of calcite cement around micrite (i.e. destruction of microporosity) is more extensive in the water zone than in the oil zone, which is possibly contributing to the lower porosity and permeability of the carbonate reservoir in the water zone. Similarity in bulk oxygen isotopic values of micritized packstones and grainstones in the water and oil zones (average δ18OV-PDB = −7.2‰ and −7.8‰, respectively) is attributed to: (1) a small difference in temperatures between the crest (oil zone) and the flanks (water zone); and (2) calcite precipitation around micrite occurred prior and subsequent to oil emplacement. Bulk carbon and strontium isotopic compositions of micritized packstones and grainstones in the water and oil zones (average δ13CV-PDB = +3.7‰ and average 87Sr/86Sr ratios = 0.707469) indicate that calcite cement was derived from marine porewaters and/or dissolution of the host limestones. The minimum formation temperatures of bulk micrite/microspar, which are inferred based on paragenetic relationships, fluid-inclusion microthermometry and oxygen isotope data, are around 58–78°C.

Diagenesis of the Khuff Formation (Permian-Triassic), Northern United Arab Emirates

This diagenetic study (including fieldwork, petrographie, fluid inclusion, and stable isotope investigations) deals with the outcrop of Upper Permian-Lower Triassic carbonate rocks, which are equivalent to the Khuff Formation. The studied succession, which outcrops in the Ras A1 Khaimah region, northern United Arab Emirates, comprises three formations, including the Bih, the Hagil, and the Ghail formations. The study focuses on unraveling the conditions and fluid compositions encountered during diagenesis of the succession. Emphasize is also made on linking diagenesis to major stratigraphie surfaces and to highlight reservoir property evolution and heterogeneity of the studied. The evolution of fluids and related diagenetic products can be summarized as follows: (1) formation of near-surface to shallow burial, fine-crystalline dolomite (dolomite matrix) through pervasive dolomitization of carbonate sediments by modified marine pore waters; (2) formation of coarse- crystalline dolomite cement by highly evolved marine pore waters (13-23 wt.% NaCl eq.) at elevated temperatures (120-208°C), and (3) calcite cementation by highly saline fluid (20-23 wt.% NaCl eq.) at high temperature (170-212°C). A final calcite cement generation has been formed by the percolation of meteoric fluids during uplift. Fracture- and vug-filling diagenetic minerals are mainly restricted to the mid-Bih breccia marker level, suggesting preferential focused fluid flow through specific stratigraphie surfaces as well as along tectonic-related structures. Reservoir properties have been evolved as result of the interplay of the original sedimentary texture and the diagenetic evolution. Porosity is higher in the Bih Formation, which is dominated by dolomitized packstones and grainstones, than in the Hagil and Ghail formations, consisting mainly of dolomitized mudstones and wackestones. Image analyses were used to quantify the visual porosity in thin sections. The highest porosity values were measured in the Bih Formation, which is characterized by significant amounts of vug- and fracture-filling cements. This feature is attributed to the increase of porosity owing to substantial dissolution of abundant intergranular and vug-filling cements. In contrast, the Hagil and Ghail formations, which consist of finer-grained rock than the Bih Formation, were less cemented, and thus, the porosity enhancement by cement dissolution was insignificant.