No abstract

No abstract

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

No abstract

No abstract

Eocene nummulite deposits along the southern Tethys margin locally constitute important hydrocarbon reservoir rocks. In order to understand the heterogeneity of these complex reservoirs, we have carried out a detailed field study of the nummulite limestones which crop out at the Kesra Plateau in Central Tunisia. The main contributions of this paper can be summarised as follows: Various species of Nummulites and planktonic foraminifera were identified in late Ypresian carbonates in the Kesra area and provided accurate biostratigraphic ages. Nummulitic limestones occur in the Chouabine and El Garia Formations representing deposition over a period of about 2Ma. Six depositional facies have been identified in the El Garia Formation, ranging from high‐energy nummulitic grainstones to laterally‐equivalent lower‐energy nummulithoclastic accumulations. Variations in nummulite morphology (size and shape) were used to distinguish a number of subfacies which correspond to different depositional environments. We provide evidence that nummulite accumulations were locally sub‐aerially exposed. A 3D reconstruction of the Kesra Plateau outcrop identified two stacked nummulite bodies, deposited under high‐energy conditions, which pass laterally into two isolated bodies in more distal parts of the platform. Regional‐scale observations indicate that the geometry of the late Ypresian nummulite platform was strongly influenced by the presence of the Oued Bahloul anticline structure, which formed a major environmental barrier between open‐marine and more restricted settings. Facies distribution in the Kesra area was strongly controlled by the inherited topography, which reflects the reactivation of NW‐SE trending faults during the Late Cretaceous and Paleocene. A tentative 3D facies model illustrates the relationship between facies and fault distributions in the Kesra area. The facies map shows that the best reservoir potential is associated with palaeo‐highs, and consists of nummulite limestones deposited in very shallow waters (a few metres) under high‐energy conditions.

The formation of shallow‐marine microporous carbonate reservoir rocks remains poorly understood in spite of their economic importance, particularly in the Middle East. In this paper, we investigate relationships between the stratigraphic occurrence of these carbonates in the Middle East and (i) the evolution of the Mg/Ca ratio in seawater; and (ii) cyclic variations in relative sea‐level. An inventory of carbonate formations in the Middle East was compiled for three geological time intervals characterised by different seawater chemistries: the Late Carboniferous to Triassic (aragonite seas); the Cretaceous (calcite seas); and the Cenozoic (transitional from calcite to aragonite seas). For each time interval, carbonate formations described as microporous have been listed. During the Cretaceous calcite sea, eleven microporous carbonate formations were deposited in the Middle East. However, no microporous carbonates were formed during the Late Carboniferous to Triassic, a time of aragonite seas. During the Cenozoic, four of the five microporous carbonate formations recorded were deposited before the transition from calcite to aragonite seas. Thus, these shallow‐marine microporous carbonates appear to have developed from precursor muds which were mainly composed of low‐Mg calcite crystals. Moreover, during the Cretaceous and the Cenozoic, microporous carbonate formations in the Middle East were generally associated with major transgressions and highstands of relative sea level. The relatively high stability of low‐Mg calcite muds may explain why shallow‐marine microporous carbonates formed during time intervals with calcite seas. In contrast to muds composed of aragonite or high‐Mg calcite crystals, the original microfabric (including intercrystalline microporosity) of low‐Mg calcite muds can partly survive moderate diagenesis.