| Marine nodular limestone reservoirs are generally developed in the Middle East, some oilfields such as
Halfaya, Ahdeb, Azadegan have reserves of more than 4 billion tons and the capacity to produce more than 400 million
tons. Strengthening the study on the genesis of nodular limestone reservoirs is of great significance to improve the
accuracy of characterization. In this paper, the research object is the reservoir of the Lower Cretaceous Khasib Formation
in Iraq A Oilfield. Based on core, thin section, logging and other basic data, this paper studies on reservoir characteristics
and sedimentary diagenesis, and clarifies the genetic mechanism of nodular limestone and its control on reservoir. The
results show that: (1) Nodular limestone reservoirs are developed in various carbonate rocks such as green algal bioclastic
limestone, micritic bioclastic limestone and bioclastic sandy limestone. Most cores of Khasib limestone reservoirs are
characterized by nodule, which can be divided into white and dark patches according to the color. The white patch
reservoir is dense and oil-free, the dark patch reservoir has good physical properties and is oil-bearing. (2) The
intergranular pores of bioclastic in white patches are almost completely cemented by bright calcite, while the cementation
of bright calcite in dark patches is weak and intergranular pores are developed. (3) In the sedimentary discontinuous
period, the form of hardground of early diagenesis is mainly divided into four stages: biological disturbance, nodule
growth, nodule fusion, and exposure dissolution. It is the main controlling factor for the formation of nodular limestone
reservoirs. The late diagenesis inherits and further strengthens the reservoir heterogeneity formed by early diagenesis. (4)
The more mature the hardground is, the more obvious the improvement of reservoir physical properties is. Among them,
the high permeability layer developed in Khasib Formation belongs to the mature hardground. After the dissolution by
atmospheric water, the pore structure is further improved. The anti-compaction effect of the hardground in the late
diagenetic stage preserves the high permeability channel in the mature hardground. This study clarifies the reservoir
characteristics and genetic mechanism of nodular limestone reservoirs, which is of great significance to improve the
characterization accuracy of nodular limestone reservoirs and formulate reasonable development (adjustment) technology
policy. |
