Abstract The current work investigates the hydrocarbon potentiality of the upper Jurassic–lower Cretaceous rocks in the Marib-Shabwah Basin, Central Yemen, through the Sabatayn-1 well. Therefore, palynological and organic geochemical analyses were carried out on 37 ditch cutting and 12 core samples from the well. Palynofacies analysis of the Madbi (late Oxfordian–early Tithonian) and Nayfa (Berriasian–Valanginian) Formations sediments indicates deposition of their organic-rich shale, calcareous shale and marl in middle to outer shelf environments under dysoxic– anoxic conditions, containing mainly kerogen of types II to III. However, the shales of the lower Sabatayn (Tithonian) Formation were deposited in an inner shelf environment of prevailing dysoxic–suboxic conditions, and show kerogen types III to II. Regional warmand relatively dry palaeoclimate but with local humid conditions developed near the site of the well is thought to have prevailed during deposition of the studied well sediments. The geochemical analyses of the Madbi Formation show higher total organic carbon content (TOC) than the overlying Sabatayn and Nayfa formations: it is varies between 1.2 and 7, and with average 4 wt% TOC, and the obtained S2 values (∼3–10, average 7 mg HC/g rock) indicates that the Madbi Formation is mainly good source rock. It shows a good petroleum potential of 4–11 mg HC/g dry rock, and the Rock-Eval pyrolysis indicates mainly kerogen types II to III (oil to gas prone) of hydrogen index values (132–258, and only one sample from Lam Member is of 360 and average 215 mg HC/g TOC). The thermal maturation parameters as Tmax (425–440 °C), production index (average 0.13 mg HC/g rock) and thermal alteration index (2 to 2+) reflected that this formation is present at margin of maturation to early mature stage oil window. So, the Lam Member (upper part) of the Madbi Formation is considered the main hydrocarbon (oil and gas) source rock in the Marib-Shabwah Basin. Accordingly, we predict that the Meem Member is an active source for gas and oil accumulations in the overlying sandstone reservoir of the Sabatayn Formation in the Sabatayn-1 well.

A detailed palynofacies analysis was carried out on 56 ditch-cutting samples obtained from the lower Cretaceous of Siqeifa 1x borehole, north Western Desert, Egypt. The main aim of this study is to identify depositional palaeoenvironments in details, reconstruct vegetation cover and to infer palaeoclimate conditions. Three palynofacies types were recognised; palynofacies type (PF-1) corresponds to the lower-middle Alam El Buieb Formation (Berriasian-Barremian). This is deposited in a deltaic (prodelta) subenvironment during a Berriasian-early Barremian regression episode, and the lower upper Alam El Buieb deposited in an inner shallow marine environment during a partial regain of a late Barremian-Aptian transgression, under prevailing dysoxic-anoxic to suboxic-anoxic conditions. The second palynofacies (PF-2) represents the uppermost Alam El Buieb, Alamein, and Dahab formations (late Barremian-Aptian), where the uppermost Alam El-Buieb Formation was deposited in a distal bar of a prograding delta, accumulated during a minor local regression. However, the carbonate of the Alamein Formation and the shale of the Dahab Formation was deposited in a saline lagoon environment developed during a partial regain of the local early Aptian marine transgression. Suboxic-anoxic to dysoxic-anoxic conditions are interpreted to prevail during deposition of the PF-2. Third palynofacies (PF-3) represents the Kharita Formation (Albian), where the lower Kharita was deposited in a lagoon setting, while the upper Kharita was deposited in a deltaic environment due to a major marine regression, under dysoxic-suboxic conditions. Local pteridophyte vegetation on low lands near the borehole and conifers on relatively dry hinterlands is interpreted to thrive under a regional warm and relatively dry palaeoclimate. Possible seasonal dry periods may be developed during deposition of the uppermost Alam El Buieb, Dahab and Alamein formations.