Black shales and oil source rocks

The objective of this study is to provide information on source organic matter input, depositional conditions and the correlation between crude oils recovered from Sunah oilfield and Upper Jurassic Madbi Formation. A suite of twenty-six crude oils from the Lower Cretaceous reservoirs (Qishn clastic) of the Masila Region (Eastern Yemen) were analysed and geochemically compared with extracts from source rock of the Upper Jurassic (Madbi Formation). The investigated biomarkers indicated that the Sunah oils were derived from mixed marine and terrigenous organic matter and deposited under suboxic conditions. This has been achieved from normal alkane and acyclic isoprenoids distributions, terpane and sterane biomarkers. These oils were also generated from source rock with a wide range of thermal maturity and ranging from early-mature to peak oil window. Based on molecular indicators of organic source input and depositional environment diagnostic biomarkers, one petroleum system operates in the Masila Region; this derived from Upper Jurassic Madbi organic-rich shales as source rock. Therefore, the hydrocarbon exploration processes should be focused on the known location of the Upper Jurassic Madbi strata for predicting the source kitchen.

Significant attention has been given, during the last decade, to Palaeozoic unconventional oil and gas shale in northern Africa where the productive Palaeozoic basins are located. New tentative
strategies shed light on Mesozoic unconventional plays represented by Cretaceous shale reservoirs.
In most petroleum systems the proven Cretaceous oceanic anoxic events (OAEs), represented mainly by the Fahdene and Bahloul Formations, played the main role in hydrocarbon generation with good distribution. Their deposition was restricted to the early Aptian (Bedoulian), the early/late Albian, and the Cenomanian–Turonian transition. Additional black shales have been detailed for the first time in Tunisia which relate to the Valanginian Weissert and late Hauterivian
Faraoni events. Biostratigraphic and complete geochemical reviews have been undertaken from published papers and unpublished internal reports to better assess these important source intervals. These black shale levels, pertaining to OAEs, were deposited  in almost deep marine
environments during short-lived periods of anoxia (ca. 2 Ma). In the course of this review, thickness, distribution, and maturity maps have been established for each level using well data from published sources.

During the Early Cretaceous important changes occurred in South-western Gondwana related to its fragmentation, and South Atlantic Ocean opening. Then, two basins developed in southernmost Patagonia: the AustralMagallanes Basin and the Rocas Verdes-Magallanes Basin. The Río Mayer Formation corresponds to the initial sedimentary stages of the Austral Basin including black shales, marls and fine-grained  sandstones deposited in an outer shelf environment. We studied and sampled three detailed sedimentary logs at Río Guanaco Locality (S 49°
57′ 11"; W 72° 04′ 56″), dividing this unit into three informal sections (lower, middle and upper). Whole rock geochemical analyses were carried out over the three sections with the focus on trace elements, in particular, rare earth elements (REE). While, the lower section of the unit shows an affinity to acid volcanic source areas, the middle and upper sections show an affinity to intermediate source areas. These changes through the Río Mayer Formation can be correlated with the initial magmatic activity hosted in the Southern Patagonian Batholith during the Neocomian. Considering the REE patterns, the lower section of the Río Mayer Formation shows an affinity to the felsic compositions of the Tobífera Formation, while the upper section would be related to the erosion of the Punta Barrosa/Cerro Toro formations of the Rocas Verdes-Magallanes Basin. The clear distinction between the REE patterns of the Río Mayer and Zapata formations is considered evidence of the compartmentalization of these basins during the Early Cretaceous. Finally, the absence of correlation between the geochemistry of the studied unit and the Jurassic volcanic rocks of the Deseado Massif indicates that the north-eastern sector of the basin was not a source area for the Austral Basin, at least during the deposition of the Río Mayer Formation in the study area.

Niutitang Formation in the South China block is considered a source of hydrocarbon as it comprised an enormous quantity of organic matter. The black rock of the early Cambrian Niutitang Formation is widely distributed in the Yangtze region, but the detailed geochemical investigation of these black rocks is still unclear. Therefore, this research discusses the detailed geochemical characteristics of the Niutitang Formation to reconstruct the paleoenvironmental conditions employing total organic carbon (TOC) content, major, trace, and rare earth element data. For this purpose, the black rock specimens of the Niutitang Formation from two outcrop sections were utilized for geochemical characterization, and the results are compared with the other eight sections from the South China Block. The average total organic carbon in these sediments is significantly higher (5.80 wt. %). At the platform region, the lower quantities of TOC indicate the poor potential to produce hydrocarbons. At the same time, significantly higher TOC is noticed at the deep shelf and slope sediments, indicating the significant potential to produce hydrocarbons. The average Ce, Eu and Y anomalies from both studied Longbizui and Sancha sections are 0.74, 0.86, 1.77, 1.07, and 1.19, 1.30, respectively. Moreover, the chemical index of alteration (CAI) throughout the Yangtze block is higher (averaging 71.32) than to Post Archean Australian Shale (PAAS 69), indicating a moderately weathered source of Niutitang Formation relative to PAAS. As the sediments are moderately weathered, it suggests these rocks might have been derived from felsic rocks, mainly from granite – granodiorite. The normalization of REEs in black rocks reveals a reduction of light REEs with elevation in heavy REEs enrichment. Similarly, positive Eu anomaly, negative Ce anomaly, and a moderate Y/Ho (34.61) are clues for the hybrid depositional mechanism associated with hydrothermal action and terrigenous input. These anomalies are also evidence of upwelling in the paleo-ocean and the mixing of organic matter, which created anoxic bottom water during the deposition of Niutitang Formation in the basin and the upper oxic water conditions before the deposition. The main controlling factors for the distribution of rare earth elements in these black rocks of the Niutitang Formation are pH, terrigenous input, source rock composition, tectonism, upwelling mechanism, and hydrothermal activities.

On the basis of an inorganic concept of the petroleum origin, the phase relationships of crystalline kerogens of black shales and liquid oil at the physicochemical conditions of a typical geobarotherm on the Texas Gulf Coast are considered. At the conditions of the carbon dioxide (CO2) high fluid pressure, the process of oil transformation into kerogens of varying degrees of "maturity" (retrograde metamorphism) takes place with decreasing temperature and hydrogen pressure. Kerogen generation in black shale rocks occurs by the sequential transition through metastable equilibria of liquid oil and crystalline kerogens (phase "freezing" of oil). The upward migration of hydrocarbons (HC) of oil fluids, clearly recorded in the processes of oil deposit replenishment in oil fields, shifts the oil ↔ kerogen equilibrium towards the formation of kerogen. In addition, with decreasing of the hydrogen chemical potential as a result of the process of hightemperature carboxylation and low-temperature hydration of oil hydrocarbons, the "mature" and "immature" kerogens are formed, respectively. The phase relationships of crystalline black shale kerogens and liquid oil under hypothetical conditions of high fluid pressure of the HC generated in the regime of geodynamic compression of silicate shells of the Earth in the result of the deep alkaline magmatism development. It is substantiated that a falling of hydrogen pressure in rising HC fluids will lead to the transformation of fluid hydrocarbons into liquid oil, and as the HC fluids rise to the surface, the HC ↔oil ↔ kerogen equilibrium will shift towards the formation of oil and kerogen. It is round that both in the geodynamic regime of compression and in the regime of expansion of the mantle and crust, carboxylation and hydration are the main geochemical pathways for the transformation of oil hydrocarbons into kerogen and, therefore, the most powerful geological mechanism for the black shale formations.

The objective of this study is to provide information on source organic matter input, depositional conditions and the correlation between crude oils recovered from Sunah oilfield and Upper Jurassic Madbi Formation. A suite of twenty-six crude oils from the Lower Cretaceous reservoirs (Qishn clastic) of the Masila Region (Eastern Yemen) were analysed and geochemically compared with extracts from source rock of the Upper Jurassic (Madbi Formation). The investigated biomarkers indicated that the Sunah oils were derived from mixed marine and terrigenous organic matter and deposited under suboxic conditions. This has been achieved from normal alkane and acyclic isoprenoids distributions, terpane and sterane biomarkers. These oils were also generated from source rock with a wide range of thermal maturity and ranging from early-mature to peak oil window. Based on molecular indicators of organic source input and depositional environment diagnostic biomarkers, one petroleum system operates in the Masila Region; this derived from Upper Jurassic Madbi organic-rich shales as source rock. Therefore, the hydrocarbon exploration processes should be focused on the known location of the Upper Jurassic Madbi strata for predicting the source kitchen.