Outcrop Samples Research Articles

Depositional Paleo-Environments of Lower Cambrian Qiongzhusi Formation in the Western Middle Yangtze Block and Its Controlling Effect on the Organic Matter Enrichment

No systematic comparative study has been conducted on the factors controlling organic matter enrichment in the different depositional environments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block, leading to a large discrepancy in our understanding. Based on organic geochemical and elemental analyses of core, outcrop, rock, and mineral samples from the slope, deep-water shelf, and shallow-water shelf, in this study, comparative analysis of the organic matter content, sedimentological characteristics, and depositional paleoenvironments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block was conducted, and the main controlling factors and models of the organic matter enrichment were investigated. The results revealed that the organic matter enrichment in the Qiongzhusi Formation was jointly controlled by redox conditions, water restriction, upwelling currents, terrigenous inputs, and paleo-productivity, but the main factors controlling the enrichment during the different periods were significantly different. (1) During the deposition of the Qiong 1 Member, the extensional rifting was strong, and the sea level was always high. The low degree of terrigenous dilution and anoxic conditions favored organic matter preservation. In this period, the upwelling currents were the main factor controlling organic matter enrichment. The paleo-productivity decreased as the intensity of the upwelling currents gradually weakened from the slope to the shelf, leading to a decrease in the total organic carbon (TOC) content and thereby a gradual decrease in the biogenic silica content of the shale. (2) During the deposition of the Qiong 2 Member, the extensional rifting weakened, and the sea level continued to drop. The upwelling currents, terrigenous input, and redox conditions were all important factors controlling the organic matter enrichment in the region. From the slope to the shelves, the conditions favorable for organic matter enrichment gradually worsened, and the TOC content gradually decreased, with the lithofacies gradually transitioning from biogenic siliceous shale to clayey shale or clayey-calcareous shale. (3) During the deposition of the Qiong 3 Member, the Yangzi Platform underwent a filling and leveling-up process, and the redox conditions played a major role in controlling the organic matter enrichment. The entire region was dominated by an oxygen-rich environment, and the conditions were no longer favorable for organic matter preservation, leading to a low average TOC content. Overall, the spatial variability of the TOC content was closely associated with changes in the depositional paleoenvironment caused by sea-level changes.

Effects of carbonated water injection on the pore system of a carbonate rock (coquina)

Abstract CO2 injection is a well-known Enhanced Oil Recovery (EOR) technique that has been used for years to improve oil extraction from carbonate rock and other oil reservoirs. Optimal functioning of CO2 injection requires a thorough understanding of how this method affects the petrophysical properties of the rocks. We evaluated pore-scale changes in these properties, notably porosity and absolute permeability, following injection of CO2-saturated water in two coquina outcrop samples from the Morro do Chaves Formation in Brazil. The coquinas are close analogues of Pre-salt oil reservoirs off the coast of southern Brazil. The effects of carbonated water injection were evaluated using a series of experimental and numerical steps before and after coreflooding: cleaning, basic petrophysics, microtomography (microCT) imaging, nuclear magnetic resonance (NMR) analyses, and pore network modeling (PNM). Our study was motivated by an earlier experiment which did not show the development of a wormhole in the center of the sample, with a concomitant increase in permeability of the coquina as often noted in the literature. We instead observed a substantial decrease in the absolute permeability (between 71 and 77%), but with little effect on the porosity and no wormhole formation. While all tests were carried out on both samples, here we present a comprehensive analysis for one of the samples to illustrate changes at the pore network level. Different techniques were used for the pore-scale analyses, including pore network modeling using PoreStudio, and software developed by the authors to enable a statistical analysis of the pore network. Results provided much insight in how injected carbonated water affects the pore network of carbonate rocks.

About the weathering effect on sedimentary rocks organic matter

To get properties and characteristics of oil and gas source rocks and for other geological aims, geologists analyzes core samples from wells and outcrop samples. But we should probably somehow correct geochemical source rock properties data we get on outcrop samples, because organic matter could change significantly due to oxidizing and weathering. This problem is very important but poorly developed in modern publications, so it attracted our interest and led to this investigation. Published information on the organic matter weathering which change it’s content and quality in sedimentary rocks is collected and summarized in this article. The changes of the kerogen (isolated from the mineral matrix) elemental composition during its long-term storage are presented. We compared the results we get in laboratory and results of other authors who dealt with natural changes of organic matter in outcrops. An attempt to interpret the results obtained from the point of view of hypergene changes in OM was made. Uncertainties that require further study and development are indicated. This work is the first step to better understanding of weathering effect on organic matter content and properties – question of great importance for making adequate oil and gas prospects estimations. This study continues the work of our foregoers and Teachers – geochemists of the Petroleum Geology Department of Lomonosov Moscow State University: N.B. Vassoevich, Yu.I. Korchagina, O.A. Radchenko, V.A. Uspensky, I.E. Leifman, A.N. Guseva, O.K. Bazhenova, T.A. Kiryukhina.

Geochemical characteristics of Jurassic sandstones on the southern margin of the Junggar Basin: Constraints on provenance and sandstone-type uranium mineralization

Jurassic strata in the southern margin of the Junggar Basin are located in the piedmont zone of the North Tianshan Mountains in Xinjiang, China. Due to multiple tectonic movements, the basin has a complicated history of structural deformation. Accurate reconstructions of the tectonic setting, provenance attributes and paleoclimatic characteristics of the Jurassic strata in the area are vital to facilitate mineral resources prospecting (such as coal, oil, natural gas and uranium) in the basin. Here we used scanning electron microscopy, major and trace elements analyses (including rare earth elements; REE), clay mineralogy and heavy mineral composition in outcrop samples, to analyze the paleoclimate, tectonic setting, source rock properties and provenance characteristics in the southern margin of the Junggar Basin during the Jurassic period. The results are as follows: (1) according to major and trace elements and clay mineral assemblages, the degree of drought increased slightly from the Early Jurassic to the Late Jurassic concomitant increase. (2) Major and minor element discrimination and REE patterns indicate that the material was sourced from continental island arc and active continental margin environments, primarily from upper crustal felsic source rocks (mainly greywacke and arkose). (3) Heavy mineral and whole rock analyses show that the provenances of the Lower Jurassic sandstones were mainly from the primarily Zaire Mountain, North Tianshan Mt.-Middle Tianshan Mt., while the Upper Jurassic sediments were sourced from the primarily North Tianshan Mountain, Zaire Mountain, Middle Tianshan Mountain.

Upper Cretaceous-Paleocene Unconventional Oil Shales in Egypt and Jordan: Insights into Organic Geochemistry

The present paper investigates the unconventional hydrocarbon potential and characteristics of Campanian–Paleocene oil shales at five outcrop sections in central Egypt and Jordan. One-hundred and seven outcrop samples were analyzed for their content of total organic carbon and total sulfur. Twenty-four samples were selected for further geochemical treatment, including Rock-Eval pyrolysis, and six samples were subjected to molecular analyses using gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS). The distribution of aliphatic hydrocarbon biomarkers indicated that most of the organic matter in the studied samples had a marine origin under anoxic to suboxic conditions. The results indicated that the upper Campanian Duwi Formation in the Quseir area of central Egypt and the Maastrichtian-Paleocene Muwaqqar Chalk Marl (MCM) Formation in the Lajjun area of central Jordan had a relatively high hydrocarbon generating potential with a TOC content of up to 20%wt of immature Type I/II kerogen. Biomarker studies of the investigated formations revealed that the origin of the organic matter is marine algae with a minor contribution of bacterial biomass. The terrigenous/aquatic ratio (TAR) indicated the marine origin of the Jordanian organic matter (MCM and Al-Hisa Phosphorite Formation formations). By contrast, the Duwi Formation in Egypt and the Amman silicified limestone Formation in the Umm Qais section of Jordan showed that these localities were affected by a more terrigenous influx to the principal marine organic matter compared with other rock units. In addition, in the Abu Tartur section the Duwi Formation showed a significant microbial reworking of organic matter reducing the actual amount of organic matter and its hydrocarbon generating potentiality. Clear correlations between nature and the preservation of organic matter is noticed, where the high organic matter mainly occurs in regional paleo-upwelling areas, strongly related to an increase in marine phytoplanktonic/algal and bacterial productivity.

Preliminary source rock evaluation, paleo-depositional environment and hydrocarbon generation potential of the cretaceous organic-rich outcrops of Mayo-Figuil River, Babouri-Figuil Basin, Northern Benue Trough (Yola arm) Cameroon: Insights from bulk geochemistry

Babouri-Figuil sedimentary basin is considered the most northerly and one of the most attractive intracontinental Cretaceous basins in North Cameroon, which contains organic-rich deposits presumed to be oil shale levels. The fieldwork carried out as part of this study enabled to delineate six lithological units mainly consisting of black shales, marls, orange clay, black to grey clays and limestones. Twenty-two samples from the studied section were subjected to bulk organic (Rock Eval pyrolysis) and inorganic (biophiles XRF-trace element) geochemistry to investigate their hydrocarbon generation potential. Field data and organic geochemical analysis of organic matter reveal that three of the six lithological units are potential hydrocarbon source rocks; these are units 1 (1.50 ≤ TOC ≤3.98%), 2 (0.47 ≤ TOC ≤1.17%) and 4 (TOC = 24.74%) consisting mainly of black to grey clays and black shales. These source rocks are discontinuously distributed throughout the studied section with a cumulative stratigraphic thickness of 275 cm. The organic matter contained in unit 1 is mainly Type II/III (182 ≤ HI ≤ 382 mg HC/g TOC) with a significant contribution in Type II kerogen. It is mature (443 °C ≤ Tmax ≤448 °C) at the base (MF-1, -2, -3 and -4) and immature (Tmax = 416 °C < 435 °C) at the top (MF-5). Unit 2 contains early-mature type III organic matter (98 ≤ HI ≤ 155 mg HC/g TOC; 438–439 °C). Organic matter in unit 4 is also early-mature and has generated mainly oil (Type I or IIS). The combination of proxies of biophile trace elements (cross plots Ni/Co vs. V/Ni and V vs. Ni) indicates that this organic matter is of mixed source, both marine and terrestrial origin, deposited under oxic-dysoxic conditions. Thus, subject to further geochemical analyses and given the pessimistic predictions from outcrop samples, these units are conventional effective source rocks in place of the early reported immature oil shale levels.

Palynostratigraphy and Palaeoenvironmental Interpretation of Outcrop Formations in Parts of Ini-Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria

The palynological study of the outcrop Formations in parts of Ini Local Government Area of Akwa Ibom State, Southeastern Niger Delta Basin, Nigeria has contributed to unraveling the palynostratigraphy and paleoenvironments of the outcrops through the use of diagnostic palynomorphs. The section is made up of carbonaceous shales, sandstones, mudstones and claystones. The conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments was used to prepare twenty outcrop samples for palynological studies. The samples produced poor to fair diversity assemblages of palynomorphs. A total of 176 palynomorphs species were recorded, with 48% pollen grains, 33% spores, 12% dinoflagellate cysts, 5% fungal spores, and 2% microforam test wall linings. Based on stratigraphically selected index taxa recovered,age determination and correlation wereestablished. The samples from Imo Formation were assigned Late Paleocene-Eocene based on Psilatricolporites Crassus, Retitricolporites irregularies, Psilatricolporites sp. and Leiotriletes adriennis with the presence of Paleocene dinoflagellate cysts such as Lejeunacysta beninensis and Selenopemphix nephroides. Ameki samples were dated Early-Middle Eocene based on Proxapertites operculatus, Retistephanocolpites williamsi, Mauritidites crassiexinius, Monocolpites marginatus and Longapertites marginatus. Ogwashi-Asaba shale samples were assigned Late Eocene-Early Oligocene due to the abundant presence of Verrucatosporites usmensis, Laevigatosporites discordatus and Retitricolporites irregularis while Eze-Aku samples were tentatively assigned Paleocene because of the presence of two pollen index fossils Proxapertites operculatus and Pachydermites diederixi. Inferred depositional conditions suggest that Imo Formation was deposited in a shallow marine or near shore brackish water estuarine environment. The Ameki Formation was accumulated in a marginal marine or near shore, brackish water-estuarine environment in the upper deltaic setting. The Ogwashi-Asaba Formation was deposited in a progradational shoreline moving between the brackish water and fresh water deltaic plain. The Eze-Aku Formation was deposited in an upper foreshore fresh water environment. The palynological study of the samples are Paleogene Formations while the paleoenvironment is in a costal deltaic to shallow-marine environment and are appropriate for hydrocarbon accumulations and exploration.

Inorganic geochemistry and hydrocarbon potential of the Voltaian Basin of Ghana, West Africa

Interest in hydrocarbon prospects of Neoproterozoic sedimentary basins is on the rise, globally. This follows significant oil and gas discoveries from these basins in Asia, North America, the Middle East, and North Africa. The Voltaian Basin of Ghana covers significant parts of the country and is largely accepted as a Neoproterozoic basin, however, unlike other Neoproterozoic basins, the Voltaian Basin has little information on its hydrocarbon prospects. The current study seeks to provide an insight into the hydrocarbon potential of the basin leveraging on the inorganic geochemistry of outcrop samples of shales obtained using the Inductive Coupled Plasma Mass Spectrometry (ICP-MS) analytical method. The paleoclimatic conditions were warm – humid and arid allowing for a weak to moderate weathering of the studied samples from Sr/Cu, and C-values, also with ICV values of >1 suggesting a probable single recycling event. The sedimentation rate was somewhat rapid in an oxic depositional environment, an inference from Ce/Tm and V/(V + Ni), V/Ni, Ni/Co, U/Th, Th/U, V/Cr values. Restricted conditions prevailed in the basin and appear to have favored primary organic matter production considering Ni/Al, and Cu/Al proxies. The organic matter preservation, however, appears not to have been favored in the oxic conditions. The proxies on paleoredox conditions, upwelling and sedimentation rate, terrestrial material input, and water depth all correlated positively with TOC, suggesting their positive effect on the TOC content on the Voltaian Basin of Ghana. Detrital material input did not necessarily affect the TOC content.

Hydrocarbon potential, organisms, and depositional environments of the Lower Cambrian shales along the southern margin of the Ordos basin, Northern China

The dark shales of the Lower Cambrian Dongpo Formation, along the southern margins of the Ordos Basin, were researched based on biomarkers and carbon isotope analyses of outcrop samples to reveal their hydrocarbon potential, organism composition, and depositional environment. Similar to the Yangtze and Tarim blocks, the Lower Cambrian in the southern margin of Ordos Basin, which is a part of North China block, also deposited high abundance marine source rocks. The shales displayed good to excellent total organic carbon (TOC) content (up to 13.75%). The crossplot of phytane/nC18 vs. pristane/nC17 and organic carbon isotopes indicate the kerogen to be primarily type I-II. The low Hydrogen index values, and high pyrobitumen reflectance (>3.0%) indicate that the Dongpo shales are currently in over-mature stage. The Dongpo shales thus exhibit high hydrocarbon generation potential and might have produced and discharged massive amounts of hydrocarbons during their geological history. The G/C30H ratios (0.17–0.29) and extended tricyclic terpanes Ratio (ETR) values (0.54–0.61) suggest sedimentary environments of stratified water and relatively moderate salinity. In addition, the Dongpo shales display low Pr/Ph (<0.63), low C30*/C29Ts (<0.44), high C35 22S/C34 22 S (>0.47), and relatively abundant elemental sulfur. These parameters indicate that Dongpo shales were formed in anoxic waters. The abundant steranes and hopanes in the Dongpo shales suggest the diversity of organisms, including eukaryotes and prokaryotes. The composition of eukaryotes (planktonic algae) is consistent with that of the Yangtze and Tarim blocks, but there are obvious differences with Oman, reflecting the heterogeneity in the global distribution of eukaryotes during the Early Cambrian. In addition, compared with the Yangtze and Tarim, the relatively contribution of prokaryote to Dongpo shale is low, which may also be an important reason for the obvious enriched of 13C in kerogen. The positive relationship between TOC and Pr/Ph and C27 St (%) suggests that high primary productivity in the paleoenvironment probably attributed to red algae. The Pr/Ph, C35/C34 values and others indexes are related to anoxic conditions, which is good for preserving deposited organic matter.

Petrography and Heavy Mineral Studies of Lokoja Formation along Mount Patti North Central Nigeria: Implication for provenance Studies

The Southern Bida basin consists of the Lokoja, Patti, and Agbaja formations. This study focuses on the Lokoja formation and samples gotten were used for petrography and heavy mineral analysis. Results of all analytical techniques employed in the study are presented here. A critical assessment of each set of results and their integration aided the proper interpretation and useful discussion. This study focuses on the Southern Bida Basin, specifically the Lokoja Formation because the exposures of Patti are poorly exposed. Outcrop samples were obtained from the exposed sections of the study area in Kabawa and Robinson street respectively. Different features were observed, such as the thickness of the exposure, the nature of the lithology, color, and sedimentary structures. The dominant lithologies in the area include sandstone and ironstone. The sandy facies are generally arkosic characterized by poorly sorted very fine to conglomeratic sandstone sequence. Laboratory investigations of samples included petrography and heavy mineral analysis. Sandstone samples were collected from the study area. Each sample was divided into two parts: the first part for petrographic studies and the second part for heavy mineral analysis.

Coreflood Effluent and Shale Surface Chemistries in Predicting Interaction between Shale, Brine, and Reactive Fluid

Summary Field and laboratory observations to date indicate that the efficiency of hydraulic fracturing, as it relates to hydrocarbon recovery, depends significantly on geochemical alterations to rock surfaces that diminish accessibility by partial or total plugging of the pore and fracture networks. This is caused by mineral scale deposition, such as coating of fracture surfaces with precipitates, particle migration, and/or crack closure, because of dissolution under stress. In reactive flow-through experiments, mineral reactions in response to acidic fluid injection significantly reduced system porosity and core permeability. The present study focuses on changes to fluid chemistry and shale surfaces (inlet and fracture walls) resulting from shale-fluid interactions and integrates these findings for an improved estimate of transport-related consequences. The pre- and post-reaction shale surfaces were examined by spatially resolved scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis. Importantly, inductively coupled plasma-mass spectrometry/optical emission spectroscopy (ICP-MS/OES) was utilized to probe the chemical evolution of the coreflood effluents. The three study cores selected from the Marcellus formation represent different mineralogies and structural features. In flow-through experiments, laboratory-generated brine and HCl-based fracture fluid (pH = 2) were injected sequentially under effective stress (up to 500 psi) at reservoir temperature (80°C). SEM-EDS results confirmed by the ICP concentration trends showed significant Fe hydroxide precipitates in the clay- and pyrite-rich outcrop sample because of partial oxidation of Fe-bearing phases in the case of intrusion of low salinity water-based fluids. Porosity reduction in the Marcellus Shale Energy and Environmental Laboratory (MSEEL) carbonate-rich sample is related to compaction of cores under stress because of matrix softening with substantial dissolution, and pore filling by hydroxides, as well as secondary barite and salts. Despite the same fluid compositions and experimental conditions used for both MSEEL samples, barite precipitation was much more intense in the MSEEL clay-rich sample because of its greater sorption capacity and additional sulfate source as well as its fissile nature with multiple lengthwise cracks. ICP tests revealed time-resolved concentration behavior in produced brine and reactive fluids that in turn complemented the pre/post-reaction SEM-EDS observations. The greatest release of metal ions into brine was in clay-rich systems indicating the importance of chemical compatibility between in-situ shale and nonequilibrated injection solutions. A thorough examination of surface and effluent data pointed to the substantial influence of formation brine in the shales, mixing of brine with fracture fluid during flow, and shale mineralogy on mineral dissolution and scale formation that significantly affect flow efficiency.

New data on the palynostratigraphy and paleoenvironments of the late Miocene (Tortonian) Quifangondo Formation in the Cabo Ledo section, Kwanza Basin, Angola

A detailed palynostratigraphic and palynofacies analysis, associated with a lithological control, was carried out on eleven outcrop samples from the Quifangondo Formation in the Cabo Ledo (Petrofina) section, Kwanza Basin, Angola. The samples yield relatively diverse and well-preserved terrestrial and marine palynomorphs. A systematic analysis of the samples enables the identification of nine spore genera and 3 species, 20 pollen genera and 13 species, and 32 dinoflagellate cyst genera and 34 species. In addition, 10 genera of other aquatic palynomorphs, such as Chlorophyceae green algae, are identified. Dinoflagellate cysts and other aquatic palynomorphs typically dominate throughout the assemblages. In this section, a combined pollen - dinoflagellate cyst zonation is used to define two palynoassociations. The first palynoassociation is middle Tortonian in age and is characterized by the presence of the pollen grain Fenestrites spinosus and the first occurrence of the dinoflagellate cyst species Selenopemphix armageddonensis. Additionally, diverse Chlorophyceae green algae dominate most of this interval samples. The second palynoassociation is assigned to the late Tortonian and is characterized by a drastic decrease in Chlorophyceae green algae, as opposed to the gonyaulacales dinoflagellate cysts. The first occurrence of the pollen grain Fenestrites longispinosus marks the transition between the palynoassociations. Lithologically, the Cabo Ledo (Petrofina) section is dominated by a claystone with an increase in the silt and carbonate components towards the upper section. The lithological data, integrated with palynostratigraphy and palynofacies, suggests that the upper part of the Quifangondo Formation was mainly deposited in an inner to middle neritic environment characterized by dysoxic conditions punctuated by periods of terrestrial inflows. Such conditions typically result from seasonal fluctuations. The age and depositional environment of the upper Quifangondo Formation inferred from this new data allows a correlation with the other Quifangondo sequences previously studied by the authors. This multi-proxy approach is important for further stratigraphic analysis with other age-controlled lithostratigraphic units in the basin. Furthermore, the improvement of paleoenvironmental and depositional models for this unit is of great importance for cross-basin correlation and future petroleum exploration plays.

Microfacies analysis and reservoir discrimination of channelized carbonate platform systems: An example from the Turonian Wata Formation, Gulf of Suez, Egypt

The channelized carbonate reservoirs played important role in the global hydrocarbon production. This study presents a new contribution to the carbonate platform system of the Turonian sediments in the Gulf of Suez. The siliciclastic fluvial channelized system incised the carbonate platform in October Field to deposit excellent reservoir unit. Also, the study interprets the facies change, depositional environment, and petrophysical characteristics of the Turonian Wata sequence, to figure out the fluvial channel reservoir distribution and discrimination using surface outcrops in addition to five cored wells. The petrographic analysis and depositional settings interpretations were conducted to the subsurface and outcrop samples. Then, the conventional core analysis measurements were combined with borehole logs for optimum reservoir discrimination, rock typing, and flow units interpretation. Besides, we studied the reservoir heterogeneity and predict permeability and reservoir rock typing in the un-cored intervals. Our finding revealed that the Wata Formation is subdivided into three main lithological units characterized by 16 microfacies. The lower and upper ones are carbonate dominated, composed of wackestones, packstones, and grainstones which is deposited in platform setting belong to lower/middle shelf and mid/outer shelf to outer ramp setting respectively. The middle unit is classified into lower and upper zones, where the lower clastic zone is composed of vari-sized sandstones, siltstone, and mudstone with some plant remains deposited in a point bar of fluvial channel and flood plains. The upper zone is composed of clastic/carbonate intercalations deposited in intertidal/shelf settings. The integration between the petrography and petrophysics enables discriminating the Wata sandstone reservoir into four static rock types (PSRT 1–4), which are controlling its reservoir quality and heterogeneity. The PSRT1 and PSRT2 have the best reservoir quality in the channelized unit and are recommended to be tracked along with October Field. The HFU2 and HFU3 are the main flow capacity in the Wata reservoir, composed of PSRT1 and PSRT2, and represented by the calcareous and ferruginous quartz arenite microfacies. This study presented a secondary dolomitized microfacies with 85% oil potentiality resulted from the incision of the fluvial channel above the aerially exposed carbonate lower unit. The research addressed the optimized petrophysical properties and reservoir heterogeneity of the channelized Wata sequences. In addition, it highlights a guidelines for further exploration and production of these reservoirs in the basin.

Chemostratigraphy and Paleoenvironment of the Miocene Organic Rich Sediments in the East Kutai Sub-Basin, Indonesia

The Miocene sedimentary rocks in Samarinda area constrains organic rich sediments, which are considered as a good source rocks hydrocarbon in the East Kutai Sub-Basin, Kalimantan. The high organic material content within the sediments is related to the dynamics of depositional environment in deltaic setting. The accumulation and characteristics of organic matter in this area may be influenced by multiple factors, under a complex physical-chemical processes. Geochemical data of major and trace elements obtained for a total 309 outcrop samples from four locations were interpreted to define chemostratigraphic and paleoenvironmental conditions (paleoproductivity, detrital influx, paleoredox and paleosalinity) responsible for organic carbon accumulation and source rocks characterization. Stratigraphic variation in inorganic geochemistry allows two chemostratigraphic packages to be defined and correlated within the Miocene sedimentary sequences. These chemostratigraphic packages are geochemically differentiated using SiO2/Al2O3, TiO2/Al2O3, Na2O/Al2O3, TiO2/Nb and Sr/Ba ratio values. The chemical alteration index (CIA) suggests that the sedimentary unit was deposited in a hot and humid climate, with moderate to intensive weathering intensity. Detrital material input proxies (Si/Al, Ti/Al) indicate that the low Si/Al and Ti/Al ratios reflect a low material input providing an increasing organic matter accumulation in the Middle Miocene. However, paleoproductivity proxies (P/Ti, Ba/Al) show the organic matter enrichment is not restrained by water column productivity, as indicated by a weak correlation between TOC and productivity index. In addition, paleosalinity index (Sr/Ba) and redox indicators (V/Cr, V/Sc U/Th and Mo/Al) indicate that the sediments were deposited in a brackish environment with dysoxic to suboxic conditions and might be the main control in the enrichment of organic matter in the study area. Thus, the detrital material influx and paleoredox conditions controlled organic accumulation and source characteristics the Miocene sedimentary sequence of the Kutai Basin.Keywords: Chemostratigraphy, Kutai Basin, paleoenvironment, source rocks.

Fracability evaluation of the upper Cretaceous Naparima Hill Formation, Trinidad

The upper Cretaceous Naparima Hill Formation, a primary source rock for conventional oil and gas reservoirs in Southern and Columbus basins, Trinidad is now considered an unconventional reservoir. Effective fracability evaluation is critical to the entire process of unconventional reservoir exploitation. At present, there exists no information on the fracability of the Naparima Hill Formation. A recent study have shown that the Formation consist of four lithofacies (siliceous-calcareous mudstone, calcareous mudstones, carbonate-rich mudstone and siliceous mudstone) that are highly brittle, implying that they are easily fractured. This is debatable because brittle rocks can have a higher fracture toughness, making them more difficult to fracture. In this study, an existing fracability evaluation model that incorporates brittleness, fracture toughness, and minimum horizontal insitu stress was used to evaluate the fracability of the four lithofacies within the Naparima Hill Formation. Through a series of laboratory testing (measurements of P- and S-wave velocities, uniaxial compression tests and Brazilian tests) of the dry outcrop samples, elastic properties (Young's modulus and Poisson's ratio), uniaxial compressive strength and tensile strength were determined. X-ray diffraction analysis (XRD) was also performed on the outcrop samples to determine their mineral compositions. Brittleness indices based on the rock elastic properties, rock strength and mineral compositions were used to evaluate the rock brittleness, and the fracture toughness was estimated from the tensile strength. The study results indicate that all lithofacies are highly brittle, which is consistent with the previous study. The fracability evaluation results showed that the siliceous calcareous mudstones and siliceous mudstones are more fracable than the calcareous and carbonate-rich mudstones. The key factors that control the fracability of the Formation were found to be the amount of quartz, and the rock strength that is influenced by calcite cementation. • This paper presents fracability of the Naparima Hill Formation mudstones • Brittleness alone is not sufficient to evaluate the fracability of mudstones • High quartz content increases the fracability of the mudstones. • High fracture toughness and tensile strength lowers the fracability of the mudstones.

Sedimentary Environment and Organic Matter Accumulation of Wufeng-Longmaxi Shales, Southwest Yangtze Platform: Insights from Geochemical and Petrological Evidence

Upper Ordovician-Lower Silurian Wufeng-Longmaxi Foration is the most developed strata of shale gas in southern China. As several shale gas wells were drilled in the mountain-basin transition zone in the Southwest Sichuan basin, the attraction was drawn by this area. While, due to the complex sedimentary environment adjacent to the Kangdian Uplift, the area favorable for organic-rich shale development is still in fog. Further study is necessary to obtain a more complete understanding of organic matter enrichment in the marginal region around Sichuan Basin. We therefore focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales. In this study, a combination of methods was applied, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. According to the mineralogical and petrological analyses, five lithofacies have been defined, including mudstone, bioclastic limestone, silty shale, dolomitic shale and carbonaceous siliceous shale. And for the geochemical characterization, the detrital influx, redox conditions and paleo-productivity have been studied. The results showed that Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment.

Diagenetic controls on porosity evolution and reservoir quality of an outcrop analogue of the Jurassic carbonate reservoirs in the Arabian Plate: An example from the Middle Jurassic Tuwaiq Mountain Formation, Central Saudi Arabia

The Middle Jurassic Tuwaiq Mountain Formation (TMF; formed of three members, from bottom to top T1, T2, and T3) represents one of the primary conventional and unconventional reservoirs to be assessed in Saudi Arabia. It is considered as a tight carbonate reservoir in the Jafurah Basin and conventional reservoir in the Arabian Basin. Here, we investigate the depositional, diagenetic, and stratigraphic impact on reservoir quality and porosity evolution of the TMF through an integrated petrographical, geochemical and petrophysical analyses on outcrop samples. The TMF has a complex diagenetic history including micritization, dissolution, recrystallization and neomorphism, meteoric cementation, evaporite precipitation, dolomitization, silicification, dolomite dissolution and calcitization. Dolomitization occurred via a hypersaline fluid mechanism, based on texture, association with evaporites, and Sr concentration. X-ray diffraction results support chert replacing evaporite as the mechanism of the two chert beds in the T3 member due to the presence of anhydrite relics in their samples. This replacement formed the necessary Ca 2+ rich fluid for dolomite dissolution and calcitization. Therefore, dolomite calcitization occurs locally and is stratigraphically controlled based on its position relative to chert beds in T3. T3 and the upper part of the T2 member contain calcitized dolomites, while the lower part of T2 and T1 contain only dissolved dolomite. As the fluid went deeper in the TMF, it became weaker. Therefore, the fluid was too weak to develop dolomite calcitization process and instead only affected T1 and the lower part of T2 by dolomite dissolution. Overall, dolomite dissolution and dolomite calcitization didn't have a high impact on porosity evolution as the TMF isn't a highly dolomitized formation. The distribution of porosity (ø), permeability (K), and reservoir-quality-index (RQI) of the TMF are stratigraphically controlled. The TMF represents the regressive system tract of a 3rd order depositional sequence, which is composed of seven 4th order cycles (i.e. deep lagoon cycles (DLC) and shallow lagoon cycles (SLC)). RQI, Ø, and K plots showed great fitting with the 4th order depositional-cycles as DLC has very poor reservoir properties (average ø = 7.7%, average K = 0.17 mD, average RQI = 0.03), while SLC has fair reservoir properties (average ø = 15.4%, average K = 2.64 mD, average RQI = 0.11). Reservoir quality contrast is controlled by two factors: (1) the reduced mud-content and increased grain-content in SLC compared with DLC resulted in increased primary interparticle porosity, and is governed by depositional energy; and (2) the higher primary interparticle porosity in SLCs compared with DLCs preferentially facilitated more infiltration of undersaturated meteoric fluids, resulting in enhanced grain dissolution. This is reflected on the higher proportion of moldic and vuggy porosity in SLCs compared with DLCs. The accurate assessment of the TMF micro-scale attributes and controls on reservoir quality in an outcrop analogue that occurs in close proximity to subsurface hydrocarbon producing intervals can provide important input for oilfield appraisal and enhanced oil recovery performance. • Cementation was the main destructive diagenetic feature of the reservoir quality. • Depositional texture controlled primary porosity and dissolution related secondary porosity. • Dolomite-Calcitization and meteoric-dissolution had a stratigraphically controlled distribution. • Dolomitization happened through hypersaline fluids like many Jurassic carbonates in the Arabian Plate. • Dedolomitization mechanism was related to the process of chert replacing evaporites.

Deep Sourced Fluids for Peridotite Carbonation in the Shallow Mantle Wedge of a Fossil Subduction Zone: Sr and C Isotope Profiles of OmanDP Hole BT1B

AbstractCompletely carbonated peridotites represent a window to study reactions of carbon‐rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87Sr/86Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87Sr/86Sr values of up to 0.7241. δ13C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ13C as low as −27‰. Based on these results, we propose that during subduction at temperatures above &gt;400°C, carbon‐rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87Sr/86Sr signature and the fractionated δ13C values of the serpentinites and listvenites in core BT1B.

Aspects of Mineral Exploration Thinking

Editor’s note: The aim of the Geology and Mining series is to introduce early career professionals and students to various aspects of mineral exploration, development, and mining in order to share the experiences and insight of each author on the myriad of topics involved with the mineral industry and the ways in which geoscientists contribute to each. Abstract Successful exploration requires an understanding of ore deposit models, the experience to recognize ore guides in an outcrop, nonlinear thinking, and some intuition. Models, using porphyry Cu deposits as examples, combine magmatic and hydrothermal processes; however, process and the results of process are different. Models provide important understanding of process but are not ore guides and do not drive discoveries; models function as rules that inhibit prediscovery exploration thinking. Results of the genetic process are recorded in descriptive models that do not reflect the considerable geologic variations existing between the hundreds of known porphyry Cu deposits. Discoveries and discovery cycles are driven by nonlinear thinking about ore guides visible in outcrop, not by genetic or descriptive models. Reality in an outcrop typically departs from generalized models. Reinterpretations that lead to drilling prospects rejected by previous exploration groups is what makes many discoveries. Increasingly, field-portable instruments for mineral and chemical analyses will add efficiencies. The most important product of early exploration work is the geologic map, defined here as a decision-making document. Mapping of ore guides in any ore-forming system invariably leads to sampling of outcrops where high grading can help geologists rig the odds in their favor. However, the objective is a highly profitable mine, not just a high-grade sample. That means the mineralization must be sufficiently continuous to build the inventory of recoverable metal required for a profitable mine, regardless of grade. High grade gets you interested, but continuity gets the mine. The principal intangible in any discovery is intuition, often described as nothing more or less than recognition, and it invariably involves experience. Perhaps the only tangible expression of intuition is displayed by individuals or teams that are unwilling to abandon a complex prospect, a behavior often described in case histories as tenacity.

Geochemical characteristics and the organic matter enrichment of the Upper Ordovician Tanjianshan Group, Qaidam Basin, China

Organic matter enrichment and preservation are significant in the formation of source rocks. The Lower Paleozoic source rocks of the Qaidam Basin have not been extensively studied. We collected outcrop samples in order to examine the sedimentary environment, evolution, and abundance of the organic matter in the “a" member of the Upper Ordovician Tanjianshan Group from the northern margin of the Qaidam Basin. We applied detailed geochemical and mineralogical techniques (including X-ray diffraction, X-ray fluorescence spectrometry, Rock-Eval pyrolysis and gas chromatography-mass spectrometry) to assess its source rock potential, paleoenvironmental setting, analyze the main controlling factors associated with enrichment of organic matter and present an organic matter enrichment model. The results show that the total organic carbon content in the “a" member is elevated, with an average value of 1.75 wt%. Additionally, the organic matter has reached a mature-overmature stage (Requ>1.8%), and the kerogen was mainly type Ⅱ1. The ratios of Sr/Cu, Rb/Sr, Cvalue and CIA reflect hot-arid paleoclimatic conditions during the deposition of the mudstone. Paleoredox indicators V/(V + Ni), δU, Ceanomaly, and Ce/La reveal the mudstones formed in an anoxic environment. Paleowater depth indicators Mn/Fe and Mn/Ti show that the depth of the sedimentary water was shallower than 250 m. Meanwhile, the Mo/TOC value and the U–Mo co-variation model indicate that it was deposited under strongly restricted conditions. The apparent hot-arid climatic conditions were not conducive for the reproduction of algae, which limited paleoproductivity. The formation of strong restricted water conditions and a stratified water column were caused by the change of paleowater depth that positively affected the organic matter accumulation. The TOC content has a good correlation with paleoredox conditions, but does not correlate with the paleo-bioproductivity index P/Ti and Ba/Al. Therefore, the present research suggests the formation of these rocks was mainly based on preservation conditions, resulting from the restricted water setting, and paleoredox conditions, which played a significant role in the organic matter enrichment.