Description And Modeling Of Cretacous Carbonate Reservoirs In Ahdab Oilfield, Iraq

The objective of this paper is the determination of hydrocarbon reservoir type and estimation of oil reserve of Ahdab Oilfield, as well as the initial input data preparation for reservoir numerical simulation. The selection of the subject is based on the need of reservoir description and modeling, Cretaceous is main target interval of the study area. Under the guidance of classical sequence stratigraphy, logging and geostatistics, the research in the sequence stratigraphic framework, tectonic characteristic, reservoirs logging recognition and evaluation, and reservoir's petrophysical parameters prediction are carried out. 3-D reservoir models are established by using the method of the Minimum Surface Tension. Reservoir characteristic are revealed, and reliable geological facts are offered to target exploration and development of the oilfield. With the basic theory of classical sequence stratigraphy, the different scale sequence boundaries have been identified by comprehensive application of core, logging, and high-resolution seismic data. The correlation framework of isochronous sequence stratigraphic is established in the area in which the third-order sequence acts as the basal unit. Consequentially, Cretaceous can be divided into two tectonic sequences which comprise 5 second-order sequences and 10 third-order sequences. The Paleogen is divided into one tectonic sequence which comprises 2 second-order sequences and 4 third-order sequences. As a result, the isochronous sequence stratigraphic framework in Cretaceous and Paleogen has been established in the Ahdab Oilfield. On the basis of establishment of the isochronous stratigraphic framework, structural models related to 13 sequence boundaries and 10 reservoirs have been completed by refined structural interpretation and velocity field analysis. The structure is a compressive long-axis anticline with low relief and extends NE-SW. There are three local structural highs which formed three traps in the area. The anticline formed in Turonian-Miocen. On the basis of 7 wells logging reservoir recognition and stratigraphic evaluation, reservoir petrophysical parameters of 5 oil-bearing intervals have been predicted by using the multi-well constrained inversion. In this paper, we proposed that changing belts of the reservoirs between anticline Ad-2 and Ad-1 is the reason that distributions of Mi4, Ru1, Ru2b and Ma1 is confined to the well Ad-1 and Ad-3 area of the anticline. Therefore, the reservoirs in the area were classified to stratiform reservoir type dominated by structure and lithologic function. With 2-D seismic data and logging data, 3-D reservoirs modeling have been achieved by applying the method of the Minimum Surface Tension. The precision and capability of prediction of reservoirs models derived from both seismic data and well logs are enhanced as compared to that derived from only well logs, and these reservoir models can be used as initial input data in reservoir simulation. Based on the reservoirs modeling, the controlled OIP 4.22×108t was estimated by Volumetric Method, As well as 4 appraisal wells and the detail prospecting belts in the area was proposed.