High-Resolution Sequence Stratigraphy of the Middle Jurassic Lower Fadhili Reservoir in Khurais Complex, Central Saudi Arabia

The Middle Jurassic Lower Fadhili Carbonate Member of the Dhruma Formation in Khurais Field is composed of two composite sequences which in ascending stratigraphic order are: Lower Fadhili composite sequence 1 (LFC1); and Lower Fadhili composite sequence 2 (LFC2). Each composite sequence is composed of four high-frequency sequences (HFS).;This study is based on detailed sedimentological descriptions of nine cored wells, a total of 370.9 meters (1,217 feet). The sequences of the Lower Fadhili reservoir are made up of five facies. The facies (in order from proximal to distal) are: skeletal coated-grain grainstone upper shoreface, peloid coated-grain grainstone and packstone lower shoreface, Thaumatoporella wackestone inner lagoon, stromatoporoid wackestone/packstone outer lagoon, and Pfenderina trochoidea wackestone/mudstone transgressive lagoon. There are two more facies have been recognized from Lower Fadhili cores and are not genetically related to the Lower Fadhili reservoir facies and sequences. These facies are: argillaceous mudstone and calcareous shale marginal marine and cemented ooid coated-grain intraclast grainstone beach facies.;The base sequence boundary of the Lower Fadhili reservoir, the base of LFC1, overlies a subaerial exposure (karst) surface on top of the ooid coated-grain intraclast grainstone beach facies that contains meteoric calcite cement, which is evidence for an exposure surface and a potential unconformity. The top sequence boundary of the Lower Fadhili reservoir (top of LFC2) is marked by a major subaerial exposure surface (karst), filled from the above by the green calcareous shale facies (reservoir seal).;Fining-upward small-scale cycles of the first composite sequence (LFC1) of the Lower Fadhili reservoir onlapped onto the unconformity. These cycles are dominated by Pfenderina trochoidea wackestone facies and stromatoporoid packstone/wackestone facies. The maximum flooding surface (MFS) can be traced across the fields and it coincides with a thin layer of Pfenderina trochoidea wackestone facies. In the highstand systems tract (HST), the abundance of Pfenderina trochoidea decreases upwards and the abundance of sponge spicules and Thaumatoporella increases upwards.;The second composite sequence (LFC2) highstand was dominated by fining-upward cycles of stromatoporoid packstone/wackestone facies, shallow marine lagoonal wackestone and shoreface coated-grain grainstone. Thaumatoporella and Cladocoropsis are abundant in the lagoonal wackestones. Toward the top of the HST of the second composite sequence (LFC2), the abundance of Thaumatoporella increases and the abundance of stromatoporoid decreases upward. Bedding in the HFS thinned upward and was capped by a subaerial exposure (karst) surface at a sequence boundary.;The Lower Fadhili was deposited in a shallow marine, tropical intra-shelf basin. The Lower Fadhili carbonate reservoir is overlain and underlain by green marls (reservoir seal).;The dominant porosity type of the Lower Fadhili reservoir facies is microporosity, except that the coated-grain grainstone and packstone facies has an interparticle porosity. The peloid coated-grain grainstone and packstone facies has the best reservoir quality because the interparticle porosity is well connected.;The use of high-resolution sequence stratigraphy of the Lower Fadhili reservoir has been beneficial to reservoir characterization and geological modeling.