Effects of dolomitization in reservoir quality of Sarvak Formation (Cretaceous) in one
of the giant oil field in SW Iran
R. Moussavi-Harami1, A. Mahboubi1 and M. Allah-Abadi2
1. Department of Geology, Ferdowsi University of Mashhad, Mashhad, Iran.
2. Department of Geology, Islamic Azad University, Shahrood Branch, Shahrood,
Iran
Sarvak Formation with Albian-Cenomanian age is one of the major oil producing
units in the Zagros Basin. In the studied wells, Sarvak is about 637 meters thick and
can be divided into two parts in ascending order: the lower part is 257 meters and is
composed of interbedded fine grained limestone with pelagic fauna and shale and the
upper part is 380 meter thick and consists of dolomitized neritic limestones with
interbedded of fine grained limestones. Based on petrographic (207 thin-sections of
cutting) and petrophysical studies of well logs, the effects of dolomitization on
reservoir characteristics of the sarvak Formation have been evaluated. According to
crystal size and shape as well as presence of dolomite in different lithofacies, three
types of dolomite have been identified. Type one is fine grained (5-60 microns) that
has formed at the early stage and type two is medium grained (60-250 microns) and
type three is coarse grained (250-500 microns) that both of them formed the burial at
the later stage of diagenesis. Based on petrography and well log analysis, the second
type dolomite has increased the quality (porosity and permeability) of the reservoir
rocks in this field.