Depositional environment and paleoclimate of the Middle Jurassic sedimentary rocks in Matruh Basin, Northwestern Desert, Egypt

This study investigates the depositional environment and paleoclimate conditions of the Middle Jurassic Khatatba Formation in Matruh Basin, northwestern Egypt, which represents an important regional gas producer in the East Mediterranean province. Khatatba Fm is marked by in situ dual source and reservoir characteristics, where fluctuations of fine and coarse clastics form intra-formational sources (lithology: shale and argillaceous siltstones), where TOC wt%: 0.88–12.98, avg. 2.95, in Matruh-5 and TOC wt%: 0.80–6.99, avg. 2.26, in Matruh-6 wells, respectively. Yet, reservoir rocks (Lithology: sandstone) were deposited during semi-arid and semi-humid conditions. This in situ dual performance is related to Tethyan sea level oscillations and climate changes during the Middle Jurassic. The Tethyan rifting that occurred during the Late Jurassic and Early Cretaceous caused subsidence events, which were accordingly associated with source rock maturation. Studying the provenance and sedimentation conditions responsible for the accumulation of organic-rich and organic-poor deposits in the Khatatba Formation in the Matruh Basin. Therefore, inorganic geochemical and geophysical data were used to interpret the depositional environment. Clastics of the Khatatba Formation were sourced by moderate weathering in the target wells of the current study (chemical index of alteration = CIA: avg. 70 and avg. 59, index of chemical variability = ICV: avg. 1.1 and avg. 1.8, chemical index of weathering = CIW: avg. 77.8 and avg. 64.7) for Matruh-5 and Matruh-6 wells, respectively. Moreover, the Rb/Sr ratios (avg. 0.36, 0.29 in Matruh-5 and Matruh-6 wells, respectively) have been used to figure out how much chemical weathering happened in the sediments' source area, which is classified as moderate weathering. The K2O/Al2O3 elemental ratios (avg. 0.14 in both wells) are below 0.4, which suggests sediment maturation. Besides, the K2O/Al2O3 ratios were also utilized to infer chemical weathering in the target wells, where values suggest a moderate level of weathering in the area where the parent rocks were situated as well. Within the Matruh-5 and Matruh-6 wells, there is a strong correlation between high levels of strontium (Sr) and low levels of calcium (Ca) in this particular sediment type of Khatatba Fm. This explains why these deposits were accumulated in tide-dominated deltaic environments based on salinity, where Sr/Ca ratios have been used as an indicator of salinity. Moreover, the negative correlations of Ti with Al in the Matruh-5 well (R2= 0.1139 U.Safa; R2=0.0007 L.Safa) suggest no association with aluminosilicate clay minerals. While the strong positive correlations of Ti with Al in Matruh-6 well (R2= 0.858 in U.Safa; R2= 0.946 in L.Safa) indicate the association of the aluminosilicate minerals. Khatatba Fm Deposition occurred during relatively semi-arid conditions (Sr/Cu: avg. 10.03; 11.33; Rb/Sr: avg. 0.36; 0.29), which were characterized by higher terrigenous inputs (sediment influx (Al/Ca): avg. 4.47, Matruh-5 well; 3.87, Matruh-6 well). Normal salinity conditions accumulated under mostly oxic to suboxic-anoxic settings, where (100*MgO/Al2O3: avg. 8.48, Matruh-5 well; 11.37, Matruh-6 well), (V/(V + Ni): avg. 0.58 in Matruh-5 well; avg. 0.32 in Matruh-6 well), and Cu/Zn: avg. 0.34 Matruh-5 well; avg. 0.27 Matruh-6 well).