Discovery of Subtle Traps in Early Cretaceous Formations of Kuwait through an Integrated Study

Azim, Saikh A.; Al-Anezi, Salah; Al-Blayyes, Mariam K.; Al-Qattan, Sarah; Al-Saad, Bader
Kuwait Oil Company, Ahmadi, Kuwait.

Aggressive exploration and development activities of this decade have led to discovery of hydrocarbons in fault controlled anticlines and stratigraphic pinchouts of Zubair and Ratawi Formations of Kuwait. A detailed study of the Lower Cretaceous stratigraphy and structural history followed by targeted drilling have established the complexity of fluid distribution and trapping mechanism.

The Zubair Formation consists of arenaceous clastics of high to transgressive systems tracts in an intensely faulted anticline. An integrated method of fault mapping from seismic signatures including coherency, amplitude and frequency volumes tied to well and production data from shallow reservoirs yielded three categories of faults for target identification: Significant, obvious but smaller and minor or indeterminate faults. Definition and mapping of quality of oil with respect to fault seal was used to identify locales of migrated oil and sweet spots of trapped oil. Trapping mechanism was identified to be genetically and tectonically controlled: migration/leaking of oil from the high stand reservoirs up-structure and along fault conduits in the channel sand sections abetted by insufficient clay smearing to form local seals. In transgressive system tracts, the thinner sands have sufficient seals to prevent oil leakage. Mapping of sands from seismic attributes within an overall sequence stratigraphic framework is observed to be useful in delineating stratigraphically controlled traps. Comparative study of trapping mechanism with dominantly oil bearing equivalent systems of adjacent field was used to construct the fault related oil leaking pattern. Localized pressure differentials were used to locate fault traps and huge reserves were added in the process.

Paleogeographic reconstruction, diagenesis and structural analysis were used for locating stratigraphic traps in Ratawi Formation. In Upper clastic unit, oil trapping in sands is controlled by stratigraphy and lithology: Porous shoreface sands are oil bearing in three strata bound layers in areas of distinct paleogeography. Northern part is devoid of oil due to intense cementation and gradation of clastics to carbonates. The abnormally pressured Limestone member is a ramp carbonate with intense cementation towards the base: Lesser connected vuggy pores in upper part contain biograded oil from early charge which was followed a lighter fraction.

The paper describes the challenges in exploring the subtle traps in detail.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012