Organic Matter Assessment And Paleoenvironmental Changes Of The Middle Jurassic Main Source Rocks (Khatatba Formation) In The North Western Desert, Egypt: Palynofacies And Palynomorph Perspectives

The Middle Jurassic in the north Western Desert, Egypt, was a time of complex tectonics and increased environmental perturbations attributed to the predominant sedimentation of organic carbon-rich fine siliciclastic and carbonate deposits of the Khatatba Formation. Although some studies have addressed the hydrocarbon potential and source rock characteristics of the Khatatba Formation, a regional-scale investigation of the prevalent paleoenvironmental conditions and organic matter characteristics is still necessary. In this study, the Khatatba Formation is investigated for detailed palynofacies analysis and palynomorph composition to assess organic matter kerogen types and reconstruct the depositional paleoenvironmental patterns on a regional scale. For this purpose, 116 drill cuttings were collected from five wells in the Matruh, Shushan, and Dahab-Mireir Basins. Moderately diverse assemblages of spores, pollen, and dinoflagellate cysts are reported. Age-diagnostic dinoflagellate cysts, including Adnatosphaeridium caulleryi, Dichadogonyaulax sellwoodii, Korystocysta gochtii, Wanaea acollaris, and Pareodinia ceratophora, along with occasional records of Systematophora areolate and Systematophora penicillate, defined a Bajocian–Callovian age. Based on particulate organic matter (POM) composition, four palynofacies assemblages (PFAs) are identified. PFA-1 is the most common within the Khatatba Formation in the five studied wells. It contains high proportions of phytoclast fragments versus low contents of amorphous organic matter (AOM) and palynomorphs and is defined by a gas-prone kerogen Type III. PFA-2 is comprised of moderate abundances of AOM and phytoclast characteristics of oil-prone kerogen Type II. PFA-3 is dominated by phytoclasts and moderate to low proportions of AOM and palynomorphs of kerogen Type III, whereas PFA-4 consists of AOM and palynomorphs defining kerogen Type II. PFA-1 indicates predominant deposition in proximal active fluvio-deltaic sources to marginal marine conditions with enhanced contributions of terrestrial/riverine influx. PFA-2 and PFA-3 reveal deposition under an enhanced dysoxic to anoxic proximal inner neritic shelf due to the abundant occurrences of spores and coastal to shallow marine dinoflagellate cysts. PFA-4 suggests deposition under enhanced suboxic to anoxic distal inner neritic conditions because of enhanced AOM and abundant proximate and some chorate dinoflagellate cysts. Thus, the Middle Jurassic experienced a predominantly marginal to shallow water column in this part of the southern margin of the Tethyan Ocean where the Matruh, Shushan, and Dahab-Mireir Basins were located

Mesozoic and Cenozoic Cooling History of the Qiangtang Block, Northern Tibet, China: New Constraints from Apatite and Zircon Fission Track Data

This study used a new set of zircon and apatite fission track ages to quantitatively document the tectonic evolution and cooling histories of the Qiangtang block of the central Tibetan Plateau. The results indicate that the Qiangtang block underwent three cooling stages at ~148 - 73, ~50 - 20, and ~20 - 0 Ma. The three-stage cooling history and tectonic exhumation were controlled by the closure of the Bangong-Nujiang Suture during the Late Jurassic-Late Cretaceous, the India-Asia collision in the Paleogene, and the underthrusting of the India Plate during the Late Cenozoic. In addition to revealing the Late Jurassic-Late Cretaceous cooling events, the annealing patterns of the zircon fission track samples indicate different burial depths, which may help identify the Jurassic basin characteristics of the Qiangtang block. The apatite fission track (AFT) ages range from 60 ¡_ 5 Ma to 26 ¡_ 3 Ma, with a mean age of 44 Ma. These ages indicate that the Cenozoic exhumation of the Qiangtang block may have started in the Eocene. Inverse modeling of the AFT data shows that the Qiangtang block had a relatively slow cooling rate of approximately 0.5 - 1¢XC Myr-1 from 50 to 20 Ma. After ~20 Ma, most of the samples show evidence for a rapid cooling stage with a cooling rate of 4 - 6¢XC Myr-1

Geochemistry of the Eocene clastic sediments (Suonahu Formation) in the North Qiangtang Basin, Tibet: implications for paleoclimate conditions, provenance and tectonic setting

The geochemistry of clastic sedimentary units of the Suonahu Formation from the QD17 Well in the northern Qiangtang basin (Tibet) was studied using various chemical analyses. SiO2/Al2O3 ratios value indicate that compositional maturity and recycling of the sediments are low to moderate. The ÎŁREE contents of the clastic sediments range from 29.28 to 191.92 ppm. The REE geochemistry of the clastic sediments suggests that these different lithological samples are derived from a similar terrigenous source and the Eu anomaly was inherited from the source rocks. The paleoclimate index (C-value) varies from 0.01 to 0.36, reflecting generally arid to semiarid conditions. In addition, Rb/Sr (~0.41) and Sr/Cu (~37.02) ratios support the idea that arid conditions prevailed during deposition of the Suonahu Formation. Sr/Ba ratios (0.48- 3.23) suggest a paleoenvironment with variable salinity. The covariation among this factor and paleoclimate indicators suggests that variations in climatic conditions exerted a primary control on salinity. The chemical index of alteration (CIA), A-CN-K ternary diagram, and low Th/U ratios indicate that the parent rocks of the clastic sediments experienced weak chemical weathering. Two multidimensional tectonic discrimination diagrams based on major elements show the Suonahu Formation deposited in a rift-related basin. The TiO2 versus Zr, La/Th versus Hf, and Co/Th versus La/Sc bivariate diagrams and multi-major elements discrimination diagram indicate that the detritus were primarily derived from felsic igneous rocks with less intermediate igneous rocks.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Mechanism of Organic Matter Accumulation in Residual Bay Environments: The Early Cretaceous Qiangtang Basin, Tibet

The controlling mechanisms for the accumulation and preservation of organic matter in residual bay environments during the transition from marine to continental settings are not well understood, although oil–gas source rocks can form in this setting. In this study, we develop a case study for the Early Cretaceous black rock series in the northern Qiangtang Basin, Tibet (i.e., the Upper Member of the Suowa Formation), by conducting a combined organic and inorganic geochemical analysis of micritic limestone, marl, and shale samples from an outcrop section. Results show that total organic carbon (TOC) contents of the studied samples are between 1.74% and 7.71%, with the organic matter being Type II/III kerogen. Of the three factors that could influence the observed TOCs and organic matter types, including paleoproductivity, preservational environment, and sedimentation rate, the preservational environment appears to be the dominant factor, independent of lithology. This is typically supported by the relatively modest covariance between redox-sensitive parameters and TOC contents, e.g., <i>R</i>² = 0.625 in the Mn/Ca-TOC diagram and <i>R</i>² = 0.690 in the U/Th-TOC diagram. This suggests that the suboxic–anoxic environment in the lagoon at the residual bay area promoted favorable conditions for organic matter preservation. In contrast, the other two factors, i.e., paleoproductivity and the rate of sedimentation, differed between three types of lithologies. For shales and micritic limestones, the effect of paleoproductivity was limited on the abundance of organic matter, and no significant effect of sedimentation rate was detected. In contrast, the paleoproductivity has a definite effect on the amount of organic matter preserved in the marls. These findings also add to our knowledge of the depositional environment that existed during the Early Cretaceous marine–continental transition in the Qiangtang Basin and further built our understanding of the potential hydrocarbon resources of the basin