Petrofacies investigations of the Cretaceous Pab Formation Rakhi Gorge Eastern Sulaiman Range Pakistan - Implication for reservoir potential

Petrologic investigations of the sixty representative thin sections from the Pab Formation within Rakhi Gorge section Eastern Sulaiman Range Pakistan have been used to characterize different diagenetic patterns, identification of primary composition and reservoir characters. Sublithic, lithic and quartzose sandstones have been the principal constituents of the formation. The processes responsible for the late-stage alteration and diagenetic processes identified during the petrographic study include compaction of lithic fragments and mud clasts, formation of quartz overgrowth structures, feldspar replacement and alteration, cementation, and replacement of grains by clay minerals other ferrous elements and diagenetic minerals. The study shows that the initial porosity has been primary intergranular, but the secondary porosity has been originated in the rocks as a result of the alteration of the primary and secondary constituents, as well as fracturing. These facies characters replicate the reservoir quality including, good, medium, low-quality, and non-reservoir. Samples displaying average total porosity greater than 10 % have been declared as good quality. While rocks samples having 7 % or greater porosity are grouped as a medium reservoir. Those samples consisting of porosity values between 1 and 4% are considered as a low quality reservoir while those samples having porosity ranges low than 1% have been called non-reservoir. Based on the investigated parameters the studied samples from the Pab Formation displayed the characters of a medium reservoir that may hold significant hydrocarbon. This different quality including good and low quality zonation can be attributed to the facies and diagenetic change in the formation.

Evolution of Sedimentary Basins as Recorded in Silica Concretions: An Example from the Ionian Zone, Western Greece

Chert concretions in thick limestone successions preserve a more complete paragenetic sequence of diagenetic minerals than their host limestone and interbedded shale. The goal of this study was to test the possible presence of a high-temperature mineralising system in the Ionian basin of western Greece. Upper Cretaceous chert nodules were sampled at Araxos, where rocks are highly faulted and uplifted by salt diapirism, and on Kastos Island, on the flanks of a regional anticline. Chert concretions have microporosity produced by recrystallisation of opal to quartz and fractures produced in the brittle chert during basin inversion. Diagenetic mineral textures were interpreted from backscattered electron images, and minerals were identified from their chemistry. Diagenetic minerals in pores and veins include sedimentary apatite (francolite), dolomite, Fe-chlorite, Fe oxide-hydroxide mixtures, sphalerite, barite and calcite. Sphalerite is restricted to Araxos, suggesting that inferred basinal fluids were hotter and more saline than at Kastos. At Araxos, the Fe oxide-hydroxide also includes minor Cu, Zn, and Ni. Whether the transported metals were derived from sub-salt clastic rocks and basement, or from enriched Mesozoic black shales, is unclear. The effectiveness of this novel approach to understanding fluid flow history in thick limestone successions is validated.

Coupling Relationship between Multistage Fluid Activity and Reservoir Abnormally High-Porosity Zones in the Songtao–Baodao Region, Qiongdongnan Basin

An abnormally high-porosity zone (AHPZ) is beneficial for petroleum exploration, especially for the deep tight reservoirs in a petroliferous basin. Because of lacking effective research methods, it is hard to analyze the formation process of AHPZs in different geological periods. From the perspective of the diagenetic fluid type and activity history, geochemical characteristics and fluid inclusions of diagenetic minerals were utilized to reconstruct the diagenetic fluid type and dynamic evolution. The ultimate goal is to study the genetic process of AHPZs in the Songtao–Baodao region of the Qiongdongnan basin, South China Sea. It was found that there are three sections of AHPZs at different burial depths, which are generally favorable for high-quality reservoirs. Moreover, it can be concluded that the AHPZs are closely related to multiple actions of various diagenetic fluids. The meteoric waters, organic acid, and thermal fluids facilitated the enlargement of porosity by dissolving minerals to form secondary pore spaces. The hydrocarbon fluids have positive effects on the preservation of pores by preventing cement from filling the pore space.

Better Understanding on Diagenetic Processes in the Low Productivity Reservoirs: An Opener Approach for Further Tambora G Zone Development Strategy

Tambora Field reservoir zone is sub-divided into D, E, F and G. Of the four zonation, Tambora G Zone has the lowest recovery factor. Based on this, the current study was intended to have better understanding of the diagenetic processes in low productivity reservoirs and its development strategy in that zone. Reduction of porosity and permeability due to different diagenetic intensity and causes are possible reasons for this condition. A new-improved petrology study was performed to understand the effects of diagenesis and the contribution of clay minerals which has led to worsening reservoir characteristics. In order to achieve better understanding of reservoir characteristics, integrated petrology analysis of petrography, XRD, SEM and CEC were performed. The results of detailed lithofacies analyses show that low productivity related to poor porosity and permeability is primarily caused by mechanical compaction and are worsened by chemical diagenesis related to isolated pore spaces, unconnected pore throats and depositional environment. Those affects the increase of diagenetic quartz-overgrowths, ferroan/iron-bearing and calcareous minerals. The occurrence of these minerals act as a pore-filling cement and are more developed in clean rather than shaly Sandstones. The most dominant diagenetic clay mineral in all lithofacies is recorded as kaolinite, followed by illite, chlorite and illite-smectite with this trend varying relative to depth and lithofacies. The effect of these factors is more intense when burial depth increases, and the depositional environment is increasingly distal as in the G Zone’s northeast area. Referring to this, and in order to minimize the risks of diagenetic minerals causing low productivity, the well placement strategy is addressed to the south and northwest compared to the northeast area. The proven result of applying this approach is the success story of drilling the TM-132 northwest well in 2019 that reached an initial gas rate production of around 3 MMscfd.

DIAGENETIC HISTORY VS. THERMAL EVOLUTION OF PALAEZOIC RESERVOIR ROCKS IN THE ILLIZI BASIN (Algeria)

<p>Unravelling the time-space distribution of diagenetic events modifying the pore network of reservoir rocks is a classical task of hydrocarbon research. Nevertheless, it is not always easy to reach that picture, as it needs to constraint a number of variables driving that distribution along the geological history of sedimentary basins.</p><p>Here we present the results of an integrated study performed on 15 samples of Palaeozoic reservoir sandstones coming from 3 hydrocarbon wells in the Illizi Basin. In particular, the topic of debate that promoted this study is the possible thermal effect on the Illizi Basin reservoirs rocks of the Cenozoic magmatic activity occurred in Hoggar dome, south of the studied region, and in other sector of the basin as pointed out by several magmatic intrusions.</p><p>The study was performed by combining: i) the reconstruction of the relative diagenetic timing obtained by petrographic observations; ii) microthermometric analyses of fluid inclusions trapped in diagenetic minerals; iii) low-T thermochronology (both Fission tracks and U-Th/H analyses) on clastic apatite grains; iv) vitrinite reflectance analysis; v) geohistory analysis of sampled wells. These data were used to constrain different thermal models, focussing in particular to the possible evidence of a Tertiary thermal overprint.</p><p>The results of the study can be summarized as follows:</p><ul><li>Several diagenetic minerals precipitated in the pore network of the studied rocks; among these, precipitation conditions for quartz, calcite, ankerite and to a minor degree feldspars were constrained through fluid inclusion microthermometric analyses;</li> <li>All these phases precipitated in a relatively narrow range of temperatures nicely correlated with burial depth of samples, from fluids with quite homogenous salinity (78-113 °C and 9.2-14.5 NaCl eq. %) suggesting a relatively limited time in which most of cements precipitated;</li> <li>The data on the thermal maturity of organic matter (vitrinite reflectance along the Mesozoic sequence, and vitrinite reflectance equivalent, mainly from chitinozoans, for the Silurian-Devonian sequence) seem to suggest a heating higher than the one currently observed. This may be compatible both with an episode of magmatic activity or with a late Cretaceous-Tertiary burial now eroded (in the wells studied, no more than 500-700m);</li> <li>Thermochronology shows a continuous burial until temperatures compatible with those observed by vitrinite reflectance although a minor thermal episode (i.e. with a temperature variation of the order of 10°C) is allowed.</li> </ul><p>Based on this integrated data set, different thermal scenarios have been tested, excluding or including a Cenozoic additional heating, in order to estimate the effect of the adopted thermal model on the age of cement precipitation in the pore system of studied reservoir rocks.</p>

Comment on: Tang et al. [2019]: A problematic animal fossil from the early Cambrian Hetang Formation, South China

Tang et al. (2019) described new specimens of carbonaceous compression fossils from the early Cambrian Hetang Formation in South China, for which they established the new taxon Cambrowania ovata Tang and Xiao in Tang et al., 2019. Tang et al. (2019) interpreted these fossils as the remains of metazoans, representing either the carapaces of bivalve arthropods, or early life-cycle stages of sponges. We contest the animal affinity of these fossils; instead, we propose that the specimens described as Cambrowania ovata are actually large Leiosphaeridia—in other words, collapsed hollow organic spheroidal acritarchs. The features described by Tang et al. (2019) all fall into the morphology of carbonaceous compressions of Leiosphaeridia with pyritized/baritized folds and compaction wrinkles. Such Leiosphaeridia are a common component of Cambrian (and older) siliciclastic deposits, and frequently exhibit such a pattern of pyritization, baritization, and encrustation with other diagenetic minerals.

Looking for the key to preservation of fossil marine vertebrates in the Pisco Formation of Peru: new insights from a small dolphin skeleton

The upper Neogene Pisco Formation of Peru is known worldwide as one of the most significant Cenozoic marine vertebrate Konservatt-Lagerstätten, even featuring cetacean specimens that retain remains of soft tissues or stomach contents. Previous works showed that biomediated precipitation of dolomite concretions around large-sized decaying carcasses was one of the most relevant processes responsible for exceptional fossil preservation. In turn, little is known about the modes of fossilization of well-preserved small-sized vertebrates, which are rather common in the Pisco Formation, but mostly do not exhibit dolomite concretions. We report on a cetacean specimen, identified as belonging to the extinct short-snouted, small dolphin species Brachydelphis mazeasi (Pontoporiidae), preserved within a late Miocene sandy deposit at the site of Pampa Corre Viento. This specimen consists of a moderately disarticulated partial skeleton exhibiting well-mineralized bones; it is not enclosed within a dolomite concretion, being however delimited by an evident dark boundary in the host sediment. Scanning electron microscopy and microanalytical investigations identify Mn-oxides and apatite as early diagenetic minerals around the skeleton. We argue that a rapid burial of the specimen was pivotal for the preservation of the bones, and allowed the early establishment of anoxic processes for degradation of organic matter. Coupled with availability of P in porewater, the reducing conditions and the lowered pH allowed precipitation of Ca-phosphate while increasing Mn solubility close to the pontoporiid carcass. Mn-oxides precipitated at the redox boundary, the latter defining the outer edge of the volume of sediment affected by altered chemical conditions due to the decaying processes. The permeability of the sediment and the small size of the carcass were possible factors unfavorable to extensive sulfate reduction, thus preventing the formation of a dolomite concretion and allowing bone phosphatization. This record emphasizes the role of conditions favorable to bone mineralization in absence of an isolating carbonate concretion, in cases of high quality preservation of small-sized vertebrates observed in the Pisco Formation. The observation of patterns in the distribution of diagenetic minerals in the sediment enclosing vertebrate remains without, or with limited carbonate concretions provides insights into early taphonomic processes.