AL-AJMI, MOHAMMED D., and ABDULLATIF Y. MOHAMMED, et. al., Kuwait Oil Company

Abstract: Distribution and Evolution of Fracture Systems in Carbonate Reservoirs of Kuwait

This paper describes fracture distribution, stress field indicators and the most likely directions of associated fractures in Cretaceous and Jurassic formations in Kuwait. The understanding of stress field orientation paves the way towards understanding the general fracture distribution and orientation.

Data from more than 35 wells distributed in 10 fields across the State of Kuwait is presented (Fig. 1). The study is based on the logs and their analyses, wellbore break/washout orientations and rose diagrams illustrate the preferred orientations from geological studies. These together with a small sample set of rose diagrams generated for outcrop sections at the Jal Az-Zor Ridge comprise the available data (Fig. 2).

Wells with processed imagery logs (Fig. 3), such as Burgan-C (Halliburton, 1995), Burgan-B, Magwa-B, Abduliyah-A, Raudhatain-A, Sabiriyah-A (Schlumberger, 1991-1995) Burgan-A, Magwa-A and Magwa-C (Western Atlas), provide rose diagrams indicating open fracture trends; those fractures that have been logged are mainly open fractures. In the Jurassic study (1995) the borehole washout data from 11 wells in Minagish and Umm Gudair fields were analyzed, and some variations in the data were observed.

From a regional standpoint Kuwait is located on the northeastern flank of the Arabian Plate. The Arabian Plate bounded by the Zagros crush zone to the northeast, Owen and Masirah transform faults to the east, Gulf of Aden rift to the southeast, Red Sea rift to the southwest and Dead Sea transform fault to the west. The Plate consists of two parts, the Arabian Shield adjacent to the Red Sea and the Arabian Shelf extending through Kuwait.

The principal movement for the Arabian Plate is towards the northeast (Beydoun, 1991) resulting in a major collision zone which is the Zagros crush zone in western and southwestern Iran. The Zagros fold belt (foothill foldbelt) occurs between Kuwait and the Zagros crush zone and the main axial trends are NW - SE.

The Arabian Shield is basement composed of mainly igneous and metamorphic rocks. The Arabian Shelf comprises the sedimentary cover of the peninsula extending from the shield to the Arabian Gulf; the Arabian Shelf sediments are variable and include Limestone, Oolite, Dolomite, Shale, Sand, Salt and Anhydrite having great differences in hardness and resistance to deformation where fracture characteristics will vary accordingly.

The main structures in Kuwait, including Kuwait Arch and surrounding structures; e.g., Abduliyah, Dharif and Khashman, also Raudhatain, Ratqa and Sabiriyah in north Kuwait, have variable axial trends inferring different trends of maximum horizontal stresses. The axial trace of Kra Al Maru, Rahiyah, Atraf, Ladira, Liyah, Dhabi and Mutriba trends and possibly trends at Umm Gudair and Wafra are mainly NW-SE. These structures are parallel and appear related to the Neogene - Zagros Foothills Foldbelt, which is still active. However the Zagros frontal fold is some 200-km east of onshore Kuwait, and the only possible evidence of Zagros structure in Kuwait is the Ahmadi Ridge. The N-S and northwestern trending structures are also parallel to the trends of basement features west of Kuwait, which are likely to be related to the Hercynian orogeny.

The study concludes that the present day maximum horizontal Stress direction in Cretaceous is NE-SW and WNW to NW in Jurassic, this suggests the possibility of stress-field decoupling across the Gotnia salt section, which separate Cretaceous and Jurassic formations.

The anomalous trends which have been observed in some wells for breakouts, fracture orientations and open joints can be explained as a result of the changes in the stratigraphic column and the variations in lithologies with differing elastic/rock mechanic properties. The local variations in stress field adjacent to larger regional structural elements (e.g., faults) or stratigraphic, and the Multi-modal data sets where more than one set of fault trends or conjugate fracture sets also could produce some anomalies in the data.

In summary, the fracture systems in Cretaceous and Jurassic in Kuwait formed as a result of a complex history involving collision of the Arabian and Eurasian Plates, combined with generation of overpressure in the case of the Marrat and Najmah-Sargelu reservoirs in the Jurassic section. Fractures formed in Jurassic time and later result from reactivation of structures that first originated during infra Cambrian, Caledonian, and Hercynian tectonic events. Fractures in Jurassic and younger strata show orientations that are consistent with the changing convergence direction between the Arabian and Eurasian plates and they reflect fault patterns mapped from 3D seismic data along the Kra Al-Maru trend. 

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England