Source Rock Characteristics Research Articles

Seismic characterization of source rocks: Why should it be assessed on a site-by-site basis?

Seismic characterization of source rocks (SRs) became widely used in exploration risk assessment, partly driven by the evaluation of petroleum systems conditioned by the presence of organic-rich SRs. Generally, rock physics combined with seismic amplitude variation with offset (AVO) analysis and inversion of seismic data are used to detect SR presence and to assess SR lateral and vertical variations measured in total organic carbon (TOC) content (in weight percent [wt%]). Despite its great potential, this method suffers from a range of pitfalls and uncertainties. In this study, based on several data sets, we highlight the variability of seismic responses of SRs. From well data, rock property studies of SRs show that the relation between acoustic impedance, which is the product of density and P-wave velocity, and TOC turns out not to be representative in SRs with TOC contents less than approximately 4–5 wt%. In the screening phase of rock-physics data, SR also reveals a large range of Poisson's ratio values, which relates to P-wave and S-wave velocities and has a direct impact on AVO. Moreover, from real seismic data, AVO analysis gave support for this complex behavior, highlighting AVO class I, III, and IV anomalies. Therefore, the expectation that the top of SR intervals would feature a “clear dimming with offset” (AVO class IV) should not be generalized for SR identification, especially in frontier areas lacking nearby well calibration, as suggested by the results of this study.

Miocene coals in the Hanoi Trough, onshore northern Vietnam: Depositional environment, vegetation, maturity, and source rock quality

The Hanoi Trough in northern Vietnam forms the landward extension of the large Song Hong Basin in the South China Sea. Several gas and condensate discoveries have been made in the offshore part while exploration in the Hanoi Trough has been less successful. Encountered hydrocarbons suggest a terrigenous source rock, the most likely candidate being Miocene coals. The composition of these is poorly documented in the public domain, but in the Hanoi Trough with a thinner Cenozoic succession numerous Miocene coal beds have been penetrated by several wells. The current study examines by organic petrography, geochemistry, and sedimentology the depositional environment, paleo-vegetation, thermal maturity and source rock potential of Miocene coal samples from two about 1000 m fully cored wells in the Hanoi Trough. The sedimentological facies, dinoflagellate cyst content and pyrite nodules suggest the coals were formed in coastal mires, in particular the younger section, testifying to an overall relative sea-level rise in the Miocene. This is supported by abundant framboidal pyrite and high TS contents in the coals. Dominance of huminite in all samples suggests water-logged mire conditions while abundant funginite and high concentrations of hopanes indicate high bacterial and fungal activity. Reduced fluorescence, oxidation rims, and elevated reflectance of resinite in a sample with concentrations of liptinite composed of resinite, cutinite, suberinite, sporinite and liptodetrinite might suggest the liptinite-enrichment was the result of degradational removal of less resistant organic matter. Biomarkers indicate the mire vegetation consisted of gymnosperms related to Podocapaceae or Cupressaceae, and perhaps Pinaceae. Presence of angiosperms is indicated by oleanane and aromatic oleananes, lupanes and ursanes. An unknown diterpane derived from Araucaria nemorosa (conifer in the Araucariaceae family) and/or Araucaria cunninghamii (pine) may be the first occurrence in a geological sample. The coals are of lignite to sub-bituminous A rank and are thermally immature with regard to hydrocarbon generation. They are today not at maximum burial depth due to c. 480–1450 m of Miocene uplift before reburial in Plio-Pleistocene time. HImax values around 200 mg HC/g TOC reveal the coals primarily are gas-prone, but exsudatinite in cleats in huminite and cell lumens in funginite is evidence for in situ generation from labile macerals. Oil droplets in cleats/cracks suggest migration of liquid hydrocarbons from deeper buried source rocks and are indicative of a working petroleum system in the Hanoi Trough.

Study on Well Logging Technology for the Comprehensive Evaluation of the “Seven Properties” of Shale Oil Reservoirs—An Example of Shale Oil in the Lucaogou Formation in the Jimsar Sag, Junggar Basin

The shale oil reservoir in the Lucaogou Formation in the Jimsar Sag, Junggar Basin has a complex lithology, complex pore structure, and strong heterogeneity. Conventional logging reservoir evaluation methods have difficulty accurately evaluating the lithology, physical properties, electrical properties, oil-bearing properties, source rock characteristics, brittleness, and horizontal in-situ stress. Starting from the logging response characteristics of complex lithologies, a multilevel, multi-logging parameter combination shale oil logging evaluation is proposed, and an evaluation model of the “seven properties” of shale oil reservoirs is established. This model is used to interpret the data in the study area and achieves good results, effectively solving the “three types of quality” evaluation problems of shale oil source rock quality, reservoir quality and engineering quality. Based on the comprehensive evaluation of the seven properties of logging, it is proposed that the lithology of the Lucaogou Formation shale oil determines the physical properties, brittleness, source rock properties, rock mechanical properties, contact relationships with the reservoir and physical property controls on the oil-bearing property; the “sweet spot” of shale oil in the Lucaogou Formation is clarified, providing effective support for the development of shale oil in the Jimsar Depression.

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.

Organic geochemical characteristics of Enping Formation source rocks in Wenchang A sag, Pearl River Mouth Basin, China

Organic geochemical characteristics of Enping Formation source rocks in Wenchang A sag, Pearl River Mouth Basin, China

Biostratigraphy and sequence stratigraphy of the deep offshore Niger Delta and implications for future hydrocarbon exploration in the Paleogene sequences

The Niger Delta Deep Offshore Basin has been the latest frontier within the Niger Delta for hydrocarbon exploration and production activities. Until the onset of drilling activities in the early 1990s, little was known about the biostratigraphy of this frontier setting. Thus, uncertainties existed in the ages ascribed to the various stratigraphic sequences. Presently, several wells have been drilled in the deep offshore Niger Delta and three-pronged biostratigraphic data and some sequence stratigraphic interpretations have become available. We have integrated biostratigraphic data comprising palynological, foraminiferal, and calcareous nannoplankton zonation and biofacies, wireline log information, and regional-3D seismic data, with simple basic sequence stratigraphic analysis for a detailed well correlation, regional mapping of hydrocarbon-bearing intervals, and evaluation of the thicknesses of promising stratigraphic intervals. Results indicate a significant effect on not only the earlier conceptualized age of the deepest prospective reservoirs, but also the ages of possible source rocks in the Paleogene of the Niger Delta offshore sequence. Our findings will help in the realistic assumptions of source rock characterization, charge modeling/prediction, and the stratigraphic thicknesses of the offshore sequences and their reservoirs. We examine the specific derivable age interpretations from the integration of at least six wells, biostratigraphic data with seismic, and the implications of these interpretations for hydrocarbon prospectivity of the Paleogene sequences. Furthermore, we document the biostratigraphy work done in the area, in all the structural belts where drilling has occurred including the deepest well drilled in the deep offshore Niger Delta, Bosi-006. The results of this study have a critical impact on the current regional geologic understanding of the deep offshore Niger Delta.

Significant contribution of haloalkaliphilic cyanobacteria to organic matter in an ancient alkaline lacustrine source rock: A case study from the Permian Fengcheng Formation, Junggar Basin, China

Alkaline lakes are productive aquatic environments, and related source rocks appear to possess considerable hydrocarbon generation potential. The Fengcheng Formation (Fm.) source rock is a good case to investigate organic matter enrichment and hydrocarbon generation characteristics of such alkaline lacustrine source rocks. However, the biological contributor to organic matter of the Fengcheng Fm. is generally controversial. Based on geochemical, biological, and theoretical evidence, a new understanding of the biological contributors to organic matter of alkaline lacustrine source rocks was reassessed. The depositional paloenvironment of the Fengcheng Fm. was reconstructed and compared to the Tumbiana Fm. (a typical alkaline lake case) by using alkaline minerals, trace elements, nitrogen isotopes, and biomarkers. The results of abundant alkaline minerals, Pr/Ph < 1, Sr/Ba > 1, B/Ga > 7, gammacerane index >0.2, and δ15N > 10‰ (published data) show that the Fengcheng Fm. was deposited in a reducing, hypersaline, alkaline lake, similar to that of the Tumbiana Fm. The biological contribution to organic matters of the Fengcheng Fm. was comprehensively investigated by biomarkers, fossils, and microbial community structure analysis. The low terrigenous/aquatic ratio, abundant 2-methylhopanes, and high αααC29cholestane-20R proportion of the Fengcheng Fm. provide geochemical evidence for a larger cyanobacteria input than land plants and algae. Identified stromatolites and Chroococcus fossils provide biological evidence for the active participation of cyanobacteria during deposition. The investigation of microbial community structures of modern alkaline lakes provides a theoretical evidence for the fact that cyanobacteria are the dominant phytoplankton in alkaline lakes. The dominance of cyanobacteria in alkaline lakes relates to the following aspects: the extreme resistance of cyanobacteria, their inorganic carbon-concentrating mechanism, their adequate sodium and carbon supply from alkaline lake water, and the inhibited growth of some species of algae within alkaline waters. In summary, haloalkaliphilic cyanobacteria are the primary contributors to organic matter of the Permian Fengcheng Fm. that was deposited in an ancient alkaline lake.

Geochemical characteristics and, origin of heavy oil in the periphery of Wenchang-B depression in Zhu III Sub-basin, Pearl River Mouth Basin, China

Heavy oil reservoirs have been discovered in the periphery of Wenchang-B depression in Zhu III sub-basin, but there is no relevant research on the geochemical characteristics and genesis of heavy oil. The characteristics of source rocks and crude oil were analyzed by various geochemical experiments. The results indicated that: (1) Second member of the Wenchang Formation (WC2) source rock has the largest hydrocarbon generation potential, WC1 and EP source rocks have similar characteristics, but EP source rocks have low maturity and Pr/nC17 greater than one; (2) Its heavy oil-density ranges from 0.91–0.97 g/cm3, with low sulfur content. The saturated hydrocarbon content in well Q4 is high, whereas it has low the non-hydrocarbon content. The carbon isotope values range from −28.2 ‰ and −25.4 ‰. Combined with the biomarker characteristics, the source of heave oil corresponds to WC2 and WC1 source rocks. Hence, there are two origins of heavy oil in the Wenchang-B depression, low-mature heavy crude oil (well Q4), and biodegradable heavy oil (well Q1). Clarifying the origin and source of heavy oil can provide more references for exploration and development.

Geochemical characterization of crude oils and potential source rocks from southern coastal Ecuador: Hypothetical petroleum systems

This research paper encompasses an organic geochemical investigation on oils and sedimentary rocks from the southern coastal region of Ecuador that examines the hydrocarbon generation and accumulation in the strata of Progreso Basin Province. This petroliferous province represents an interesting area for an oil geochemistry study due to its non-elucidated petroleum systems. The present work focuses on the analysis of eighty-five oils and forty rock samples to determine kerogen types, depositional paleoenvironments, levels of thermal maturity, and in-reservoir petroleum biodegradation stages as well as to assess correlations between source facies and oil samples. Potential source rocks were selected and analysed from different outcrops and wells throughout the basin and the southern Gulf of Guayaquil. Classic biomarker, extended diamondoid, and isotope studies indicated that almost all oils from the Progreso Basin Province underwent biodegradation and were generated from multiple Tertiary source rocks deposited in a marine setting with significant influx of terrestrial organic material. Three families of oils were identified. Oil-source rock correlations suggest that the Socorro, Dos Bocas and San Eduardo formations can be considered as contributors to the studied oils. Geochemical data and previous thermal models point towards two probable hydrocarbon kitchens defined in the deepest parts of the basin as well as a third possible kitchen extended offshore beyond the Santa Elena Peninsula.

Biomarkers geochemistry of the Alpagut oil shale sequence: an evaluation of dispositional environments and source rock potential from Dodurga-Çorum basin (N-Turkey)

The Cenozoic Çankırı-Çorum basin, with sedimentary facies of varying thickness and distribution, contains raw matters such as coal deposits, oil shales and evaporate. Source rock and sedimentary environment characteristics of the oil shale sequence have been evaluated. The studied oil shales have high organic matter content (from 2.97 to 15.14%) and show excellent source rock characteristics. Oil shales are represented by very high hydrogen index (532–892 mg HC/g TOC) and low oxygen index (8–44 mgCO2/g TOC) values. Pyrolysis data indicate that oil shales contain predominantly Type I and little Type II kerogen. The biomarker data reveal the presence of algal, bacterial organic matter and terrestrial organic matter with high lipid content. These findings show that organic matters in the oil shales can generate hydrocarbon, especially oil. High C26/C25, C24/C23 and low C22/C21 tricyclic terpane, C31R/C30 hopane and DBT/P ratios indicate that the studied oil shales were deposited in a lacustrine environment, and very low Pr/Ph ratio is indicative of anoxic character for the depositional environment. Tmax values from the pyrolysis analysis are in the range of 418–443 °C, and production index ranges from 0.01 to 0.08. On the gas chromatography, high Pr/nC17 and Ph/nC18 ratios and CPI values significantly exceeding 1 were determined. Very low 22S/(22S + 22R) homohopane, 20S/(20S + 20R) sterane, diasterane/sterane and Ts/(Ts + Tm) ratios were calculated from the biomarker data. Results of all these analyses indicate that Alpagut oil shales have not yet matured and have not entered the oil generation window.

Paleoenvironment of the Lower Ordovician Meitan Formation in the Sichuan Basin and Adjacent Areas, China

The quality of hydrocarbon source rocks is affected by the sedimentary paleoenvironment. A paleoenvironment with anoxia and a high paleoproductivity is beneficial to source rocks. The paleoenvironment of the Lower Ordovician Meitan Formation of the Sichuan Basin and its adjacent areas is lacking, restricting the oil and gas exploration of the Ordovician in the Sichuan Basin and its adjacent areas. In this paper, the content of major and trace elements of 50 samples was tested to clarify the paleoenvironment of the Meitan Formation. The paleoclimate, paleosalinity, paleoredox, and paleoproductivity during the deposition of the Meitan Formation were analyzed. The control effect of the paleoenvironment on the development of source rocks was clarified, and the favorable paleoenvironment for source rock development was pointed out. The results show that the paleoenvironment of the Meitan Formation has the following characteristics: humidity, brackish water, oxygen depletion, anoxia environment, and high paleoproductivity. These characteristics are conducive to the development of poor and moderate source rocks. The source rocks of the Meitan Formation were developed in the north, west, and south of the Sichuan Basin and its adjacent areas. The organic matter of the source rocks is mainly typed II1 kerogen, and the quality is evaluated as poor-medium source rocks having the potential of generating oil and gas. This study can provide fundamental parameters for the further exploration of Ordovician petroleum.

Mapping the maturity of organic-rich shale with combined geochemical and geophysical data, Draupne Formation, Norwegian Continental Shelf

The evaluation of the interplay between kerogen maturation and total organic carbon (TOC) content within an organic-rich shale is critical to characterizing source rock shale's elastic behavior. We analysed a comprehensive database of twenty-nine well logs, two 3D seismic surveys, and geochemical data of eighteen wells within the Draupne Formation in the Norwegian North Sea. The Upper Jurassic (Kimmeridgian) Draupne Formation Shale is found throughout the Norwegian Continental Shelf and is the primary source, carrier, and seal rock in several oil and gas fields. The dataset explores a broad range of depths (2–5 km), total organic contents (0–20 wt%), and various maturation states ranging from immature to overmature. We study the relationship between rock brittleness and organic content as shale matures. We use calculated values of Young's modulus and Poisson's ratio. Academic and industry-standard cutoffs are applied to both the TOC content and maturation level. We introduce here the Organic Maturation Product (OMP) rock physics template, which incorporates a relationship between organic content and any subsequent maturation. The degree of brittleness or ductility is impacted directly by the kerogen maturation process, with lower TOC content having a dampening effect on the transition towards increasing brittleness. This template allows one to classify rock maturation in nine categories, from low to high maturation, and link these categories to rock geomechanical properties. Seismic inversion of the 3D surveys and mapping utilizing the OMP classification reveal how large-scale depositional environment and subsequent diagenetic events influence both placement and quality of source rock broadly within the basin context of the North Sea.

Evaluation of Source Rock Quality in Chang-7 Member of Major Oil Fields in Ordos Basin

Petroleum is an important natural resource for human. The demand for petroleum in the next several decades will continue to be high, which requires the exploitation of more petroleum sources. Chang-7 member of the Ordos basin is proven to be the richest among all members, and this research took a focus on making a comparison between the source rock quality of Chang-7 member in four major oil fields in the basin, to provide a simple view of priority oil fields for exploitation in the future. The research analyzed the source rock quality of four oil fields from the TOC, A%, rock type, R0, sedimentary environment, and kerogen type, and found a superiority of source rock quality of Maling and Zhidan oil field in multiple dimensions, mainly triggered by their sedimentary environment. The research concluded that the oil fields that was once in the central-southern region of the basin contains a slightly better source rock quality among the chosen fields, which is related to a more stable sedimentary environment in the early Triassic.

Characteristics of effective source rocks of Paleogene in Jiyang Depression

Characteristics of effective source rocks of Paleogene in Jiyang Depression

Organic Geochemical Characteristics of Saline Lacustrine Source Rocks: A Case Study from the Yingxi Area, Qaidam Basin, China

Organic Geochemical Characteristics of Saline Lacustrine Source Rocks: A Case Study from the Yingxi Area, Qaidam Basin, China

Evaluation of Petroleum Source Rock Using Well Logs in the Niger Delta Region of Nigeria

Petroleum source rock potential evaluation was carried out on eight wells of the Niger Delta’s Agbada Formation by assessing some source parameters from well logs and through well logs analysis based on organic matter effect and relationship on and with well logs. Source rock determinant parameters such as volume of shale (Vsh), total organic content (TOC), and amount of generated hydrocarbon (S2) was derived from well logs following established organic matter effects on well logs using Schlumberger's Techlog 2015 software, while hydrogen index (HI) was calculated using known values of TOC and S2 respectively. Well log data used for the analysis includes GR log, resistivity, sonic and Density logs. Apart from deriving source rock parameters, the well logs were also used to identify, quantify and somewhat determine mature zones of source rock intervals by correlation with TOC, S2, and HI source parameters respectively, Therefore, a source interval criterion composing of high GR log, high resistivity, high sonic and low-Density was developed alongside source rock parameters. Results of the analysis in the area show poor to fair source rock quality rating in terms of TOC, and insufficient source rock thickness fit for sustaining a world-class petroleum province like the ND petroleum province and further depicting that the Agbada FM may not be the principal source rock of the area. It is, therefore, recommended that more elaborate studies covering greater depths of the Agbada FM be carried out in other to properly represent the true source quality of the area.

Machine-Learning Techniques Characterize Source-Rock Images at the Pore Scale

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208632, “Reconstruction and Synthesis of Source-Rock Images at the Pore Scale,” by Timothy Anderson, SPE, Stanford University. The paper has not been peer reviewed. Nanoimaging techniques for characterizing pore-scale structure of shales trade off between high-resolution/high-contrast sample-destructive imaging modalities and low-contrast/low-resolution sample-preserving modalities. Acquisition of nanoscale images also is often time-consuming and expensive. In the complete paper, the author introduces methods for overcoming these challenges in image-based characterization of the fabric of shale source rocks using deep-learning models. Introduction A promising application of data-driven scale-translation techniques is nanoscale imaging. This application is important for studying shales because of the importance of nanoporosity in shale gas storage. Nanoimaging techniques, however, present specific challenges and can result in small image data sets that do not allow for accurate quantification of petrophysical or morphological properties. Consequently, data translation and generation both offer many opportunities to assimilate multiple nano- and microscale modalities and to overcome limitations of nanoimaging systems. The author proposes an image-based characterization work flow (Fig. 1) for data-driven scale translation that uses deep-learning image synthesis and translation models to assimilate multimodal, multiscale, and data-scarce source-rock images for predicting petrophysical and morphological properties. A central challenge in source-rock characterization addressed in this work is the reconstruction of 3D volumes when only 2D training images are available. Image-translation models are presented for reconstruction of 3D image volumes from 2D training data and a porous media image-synthesis algorithm that generalizes to 3D grayscale and multimodal volume generation from 2D training data. The complete paper describes the translation and synthesis models, applies these models to source-rock image data sets, and discusses extensions and future directions for the introduced work flow.

Continental shale gas dynamic enrichment and evolution over geological time

Shale gas content alters with geological time, which indicates that its formation has evolved and is dynamically enriched. In this paper, a numerical model is proposed to describe the dynamic enrichment and evolution process of shale gas over geological time. The model involves the coupled process of geological conditions (temperature and pressure), source rock quality (abundance, type and maturity), and reservoir characteristics (mineral composition and pore characteristics). By comprehensively considering the gas occurrence state, pore water content and overpressure, we divide the shale gas dynamic evolution process into six stages: dissolved gas stage, adsorbed gas stage, free gas stage, overpressure formation stage, overpressure maintenance stage, and overpressure reduction stage. As a case study of continental shale gas from the Shahezi Formation in the Xujiaweizi Fault Depression, the dissolved and adsorbed gas stages are of less significance to shale gas accumulation, despite their long life. In contrast, the free gas and overpressure formation stages with short geological times are of decisive significance for the efficient accumulation of shale gas, whereas the overpressure maintenance and reduction stage are of great significance to shale gas preservation. Moreover, the gas-in-place content simulated by this method is close to the estimated value of the process analysis method, which is higher than that estimated by the United States Bureau of Mine method and lower than that estimated by the Amoco curve fitting method.

Sedimentological, petrophysical, and geochemical controls on deep marine unconventional tight limestone and dolostone reservoir: Insights from the Cenomanian/Turonian oceanic anoxic event 2 organic-rich sediments, southeast Constantine Basin, Algeria

The Cenomanian/Turonian period is characterized by the deposition of organic-rich and fine-grained sediments that form important source rocks and unconventional reservoir units in North Africa and other parts of the world. The Lower Turonian deposits of the Djebel Darmoun Field, southeast Constantine Basin, Algeria, comprise a thick carbonate succession that has potential as an unconventional reservoir unit. This research aims to investigate the Lower Turonian succession in terms of microfacies, diagenesis, petrophysical, and geochemical characteristics, which were previously unknown. Sedimentological and diagenesis analyses were performed based on the core and thin section observations. Furthermore, geochemical and petrophysical analysis methods were carried out to assess the resource potential based on geochemical measurements, core measurements, and well logging evaluation in an arrangement. Based on sedimentological investigation, the Lower Turonian carbonate section is composed of tight limestone and dolostone rocks characterized by four bioclastic limestones and two dolomicrite microfacies, while paleontological investigation indicates that the faunal content is dominated by planktonic assemblages, revealing a pelagic deep marine setting environment. The primary pores of the Lower Turonian carbonate rocks are seldom preserved due to multi-stage diagenetic alteration (i.e., cementation, compaction, fracturing, dolomitization, dissolution, asphaltene), while dominant secondary pores are represented by dissolution-enlarged and open fractures/fissures, while intercrystal pores might be added in dolostone intervals. Dissolution and fracturing diagenetic events play a major role in enhancing the pore network, while compaction, cementation, and asphaltene diagenesis processes share in the obliteration of the petrophysical properties. Dolomitization played a dual role in the dolostone intervals, where it tended to develop the porosity at the early stages of diagenesis, while it tended to decrease the porosity at the latest diagenesis stages. The geochemical analysis indicated high amounts of organic matter in these intervals, where the higher estimated organic richness reaches a maximum TOC of 7.8% at the lowermost part. The petrophysical analysis inferred that the Lower Turonian carbonate sediments are characterized by low porosity (avg. 4%), very low permeability (<0.01mD), with dominant micro and meso porosities, and therefore they can be classified as tight carbonate unconventional resources. This study provides an insight into factors controlling the pore system network (microporosity) of fine-grained carbonate rocks as an excellent analog of the deep marine unconventional tight limestone and dolostone reservoirs and improves the knowledge and modeling of these unconventional resources. The study sheds light on the organic-rich sediments that are associated with oceanic anoxic events (OAEs) across the North African region and other parts elsewhere, forming high quality source rocks that might be neglected as excellent unconventional resources.

Organic geochemical and petrographical characteristics of the major lower cretaceous petroleum source rock (Makhul Formation) in Kuwait - Arabian Gulf

The Lower Cretaceous Makhul Formation is one of the major petroleum source rocks in Kuwait. This study aims to evaluate the Makhul source rock for its organic matter richness and its relation to the rock composition and depositional environment. A total of 117 core samples were collected from five wells in Raudhatain, Ritqa, Mutriba, Burgan, and Minagish oil fields north and south Kuwait. The rock petrographical studies were carried out using a transmitted and polarized microscope, as well as SEM and XRD analyses on selected samples. Total organic matter TOC and elemental analyses were done for kerogen type optically. The GC and GC-MS were done as well as the carbon isotope ratio. The results of this study show that at its earliest time the Makhul Formation was deposited in an anoxic shallow marine shelf environment. During deposition of the middle part, the water oxicity level was fluctuating from oxic to anoxic condition due to changes in sea level. At the end of Makhul and the start of the upper Minagish Formation, the sea level raised forming an oxic open marine ramp depositional condition. Organic geochemical results show that the average TOC of the Makhul Formation is 2.39% wt. High TOC values of 6.7% wt. were usually associated with the laminated mudstone intervals of the formation. The kerogen is of type II and is dominated by marine amorphous sapropelic organic matter with a mixture of zoo- and phytoplankton and rare terrestrial particles. Solvent extract results indicate non-waxy oils of Mesozoic origin that are associated with marine carbonate rocks. The formation is mature and at its peak oil generation in its deepest part in north Kuwait.