| Bioturbation can change the original sedimentary fabric of carbonate rocks, and affect later diagenesis, which lead to the difference of physical properties between burrows and matrix, and enhance the heterogeneity of carbonate reservoirs. The Khasib Member 2 (Kh2) of Upper Cretaceous in AD Oilfield, Iraq was mainly developed in middle-outer carbonate ramp with upward-shallowing cycle, where bioturbation relic fabrics developed widely. Based on the analysis
of a large number of core, thin section, point permeability and other test and analysis data, this paper studies the types of bioturbation relic fabric and its influence on reservoir heterogeneity. The results show that: (1) There are mainly three
types of relic fabrics: Thalassinoides, Ophiomorpha and Paleophycus in Kh2. The outer wall of Thalassinoides burrow is
smooth without lining structure, which is in the shape of three-dimensional network connected tube, and the straight
diameter of a single burrow is 5-25 mm; Ophiomorpha burrow is "T" shaped tubular, mostly horizontally distributed, and
partially developed argillaceous lining structure, with a diameter of 5-15 mm; Palaeophycus burrow is a circular-oval
slightly curved-straight pipe with a diameter of 2.5-10 mm, which is mainly distributed horizontally without lining
structure. (2) The core has obvious patch-like characteristics due to the oil-bearing difference caused by the difference of physical properties. At the same time, the regional distribution of different oil-bearing levels is related to the morphology and spatial distribution of biological relic fabric, which indicates that the reservoir heterogeneity of Kh2 is mainly controlled by biological disturbance. (3) Different bioturbation relic fabric types are developed in different sedimentary
environments of Kh2, resulting in changes in the original fabric and chemical properties of sedimentary media between the burrow and the matrix, forming the differences in pore structure between burrow and matrix. On this basis, the later
diagenetic differences (cementation, dissolution, compaction, etc.) are superimposed, resulting in different reservoir
heterogeneity types in Kh2.
Thalassinoides relic fabric is mainly developed in green algae shoal of Kh2-3. Due to the change of chemical
properties of sedimentary media in the burrow, a large number of mold pore are formed by quasi syngenetic dissolution,
while the matrix is strongly cemented in the early stage, which intensifies the physical property difference between the
burrow and the matrix. Ophiomporpha relic is mainly developed in mediun-low energy shoal of Kh2-2. Intergranular
pores are developed in the burrow due to particle filling, and the mortar content in the matrix is high, which reflected the
reservoir heterogeneity. Palaeophycus relic fabric is mainly developed in granular shoal of Kh2-1. The content of
particles in the burrow is high, and the early cementation in the burrow is inhibited due to the change of chemical
properties of sedimentary media. With the decline of sea level, atmospheric fresh water preferentially enters the burrow channel for dissolution. |
