Paleo-productivity and petroleum source evaluation of the Nkporo and Awgu shales, Lower Benue Trough, Nigeria: Insight from inorganic geochemistry

The petroleum paleo-productivity and generative potential of the Nkporo and Awgu shales in the Lower Benue Trough (LBT) were studied. Major oxides were determined using Spectro Ciros Inductively-coupled plasma optical emission spectrometer (ICP-OES) while trace and rare earth elements were determined using an ELAN 9000 Inductively-coupled plasma mass spectrometer (ICP-MS). The shales have Co/Ni ratios greater than 0.1 suggesting oil source rocks with more marine source input due to brief marine incursion in the transition zone after deposition. The very low Ni values and moderate V values, and a lack of correlation between the TOC and sulphur content, suggests that the organic matter is of terrestrial Type Ⅲ;and the very low paleo-productivity (Ba/Al and P/Ti ratios) suggests that the organic matter is of terrestrial origin. The low V/Ni values of the Nkporo and Awgu shales (Av. ≈ 4) and the high metal concentrations indicate a matured status. From the K/Rb ratios of the Nkporo (191–259) and Awgu shales (198–261), it can be deduced that the organic matter within the shales experienced a considerable loss of K and the shales are mature to generate gaseous hydrocarbons.

Paleo-productivity evolution across the Permian-Triassic boundary and quantitative calculation of primary productivity of black rock series from the Dalong Formation, South China

The change of the primary productivity across the Permian-Triassic boundary （PTB） remains controversial. In this study, records from two deep-water sections in South China （Xiakou and Xinmin sections） show the primary productivity decreased gradually from the latest Permian to the earliest Triassic, and five evolutionary stages Increase-Decrease-Recovery-Recession- Stagnation） can be observed from Clarkina changxingensis-C, deflecta to Hindeodus parvus-lsarcicella isarcica zones. Pri- mary productivity decreased abruptly from the base of C. meishanensis zone. Besides, for adjusting to the deterioration of the oceanic environment, the primary producers in the oceanic surface had changed to acritarch and cyanobacteria, which were more tolerant of stressful environment. Then the producers were under huge stagnation in the H. parvus-L isarcica zone. The values of quantitative calculation of the primary productivity from the black rock series in the Dalong Formation were very high, corresponding to that of an upwelling area in modern ocean, which shows that the strata of the Dalong Formation in the study region are potential hydrocarbon source-rocks. This result may come from the fact that South China craton was located at the equatorial upwelling area during the Permian-Triassic transitions. But organic matter contents were different in various sections because they could be affected by redox conditions and diagenesis process after burial.

Changhsingian radiolarian fauna from Anshun of Guizhou,and its relationship to TOC and paleo-productivity

Abundant siliceous micro-fossils were found from the Dalong Formation,Xinmin section,Guizhou,which include nine species of radiolarians belonging to seven genera as well as nine types of siliceous spongy spicules.The radiolarian fauna consists of all spherical radiolarians,without any species of Albaillellaria and Latentifistularia,indicating a relative shallow water sedimentary environment.The high relative co-efficient between the radiolarian abundance and total organic carbon(TOC)shows that the radiolarian assemblage represented by Copicyntra and Paracopicyntra can be used as a proxy to the paleo-productivity.However,the relative co-efficient between biogenic silica and the radiolarian and sponge spicule abundances is low.A possible explanation is that an oceanic circulation has changed the biogenic silica content after radiolarian test and siliceous spongy spicule were dissolved.

Indian monsoon variability in the Mahanadi Basin over the last two glacial cycles and its implications on the Indonesian throughflow

The orbital-scale variability of the Indian summer monsoon(ISM)has been influenced by multiple factors,such as atmospheric CO_(2)concentration,global ice volume,and insolation.Proxies for weathering activity and paleo-productivity provide potential insights into the driving forces of its variability.We documented multi-proxy data at IODP Site U1445,located in the Mahanadi Basin of the northwestern Bay of Bengal,to find out ISM variability over the last 200 ka.The proxy records,such as Nd/Sr isotopes of detrital particles,clay mineral compositions of the fine-grained sediments,biogenic opal and CaCO_(3),organic carbon contents,and carbon isotopes of organic matter,represent sediment sources,weathering patterns,and paleo-productivity related to the ISM variability.Detrital Nd/Sr isotope data and clay mineral compositions suggest that the sediments at Site U1445 originated mainly from the Ganges,Brahmaputra,and Meghna rivers without dramatic provenance change between the glacial and interglacial periods.The weathering activity inferred from clay mineral compositions and the paleo-productivity shift reconstructed by biogenic opal and CaCO_(3)contents suggest that the land-sea interactions were closely linked to the ISM precipitation between the glacial and interglacial periods.High precipitation by the strong ISM resulted in intense chemical weathering and dominant biogenic opal deposition during the interglacial periods.In contrast,low precipitation by the weak ISM led to reduced chemical weathering and predominant CaCO_(3)deposition during the glacial periods.Further,the ISM variability driving the land-sea interactions in the Mahanadi Basin was modulated by the Indonesian throughflow(ITF).Our study emphasizes the role of low-latitude forcing of climatic changes in the strong relationship between the ISM and ITF over orbital periods,providing a base for future investigations.

Petrologic and geochemical characteristics of high-quality saline lacustrine source rocks in western Qaidam Basin

The main oil source in western Qaidam Basin is the Paleogene saline lacustrine source rocks.Traditionally,it was considered that these source rocks were characterized by low abundance and poor type of organic matter as well as limited oil generation potential,but this poor source rock condition was very inconsistent with good hydrocarbon exploration results.A previous study has just confirmed that this area develops high-quality source rocks,which is of great significance for recognizing potential of hydrocarbon resource and consolidating confidence to search large and medium-sized oil and gas fields in western Qaidam Basin.Based on fine geochemical analysis of source rocks,petrology,element geochemistry and organic geochemistry are applied to discuss development conditions of high-quality source rocks in this area.The results showed that the saline lacustrine sediments in western Qaidam Basin were deposited in relatively shallow and quiet water with relatively high salinity and low nutrient content;influenced by the drought and cold climate,the terrigenous supply of fresh water was limited,and few aquatic organisms were developed.Good preservation condition is favorable for rapid preservation of oil generating biomass(such as aquatic organisms),to form source rocks rich in hydrogen element with high soluble organic matter content;but relatively lower paleo-productivity makes organic matter abundance lower than that in the eastern saline lacustrine basins.The semi-deep lake,with relatively low salinity and abundant nutrients,is the most favorable are for development of high-quality source rocks;the source rocks have large thickness in the sedimentary depression areas,but due to high water salinity and far away from the provenance area,the water stratification was obvious,so it have moderate to high organic matter abundance.

Evaluation of Organic Matter Richness of Eocene Strata Based on Calcareous Nannofossils and Rock-Eval Analysis in North Dezful, Iran

Hydrocarbon source potential of the Paleogene Pabdeh Formation was studied by means of organic geochemistry and distribution of calcareous nannofossils.Based on the results,an Eoceneaged organic matter(OM)-rich interval was identified and traced across different parts of the North Dezful zone and partly Abadan Plain.In order to characterize the OM quality and richness of the studied intervals,Rock-Eval pyrolysis and nannofossils evaluation were performed,and the geochemical data collected along selected wells were correlated to capture the variations of thickness and source potential of the OM-rich interval.Accordingly,remarkable variations were identified within the depth ranges of 2480–2552 m and also 2200–2210 m,which were attributed to the maximum increase in the rate of growth R-selected species.This increase in the productivity rate was found to be well correlated to high Rock-Eval total organic carbon(TOC)and hydrogen index(HI)values.Given that the maturity of Pabdeh Formation in the studied area was found to have reached the oil window,we expect significant hydrocarbon generation(Type II kerogen),making the play economically highly promising.