| The Cenomanian-Lower Turonian in southeastern Iraq is characterized by thick bioclastic limestone, with
complex stratigraphic structure and nested sequences of different levels. The sequence recognition is important for
stratigraphic division and reservoir correlation. Thick bioclastic limestone reservoir is commonly developed by separated
waterflooding, and the study of sequences can lay a geological foundation for the division of development units. Taking
six giant oilfields as example, all of which developed thick bioclastic limestone as main pay formations in the
Cenomanian-Lower Turonian, the sequence schemes of Cenomanian-Lower Turonian are summarized. This paper adopts
the scheme of dividing the Cenomanian-Lower Turonian into four of a third-order sequence corresponding to the four
maximum flooding surface (K120, K130, K135, and K140). Mechanisms and characteristics of key sequence boundary
such as unconformities, leaching and dissolution, thin carbonaceous mudstone, facies transition, facies mutation,
maximum flooding surface, and hardground are summarized. Southeastern Iraq is located in the northeastern margin of the
Arabian Plate, and during the Cenomanian-Early Turonian it is in a stable passive continental margin environment. This
paper uses the quadratic model of classical stratigraphy to study the sequences of thick bioclastic limestone. According to
the sequence theory, the sequence model of slightly rimmed carbonate ramp is established based on the depositional
setting of Cenomanian-Lower Turonian. Different paleogeographic locations have different water depths during the
process of sea-level rise and fall, so carbonate deposition rates and petrological features have different sensitivities to sealevel changes at different location. The water background and paleogeography control the sedimentation of different
oilfields in southeastern Iraq. The sequence and depositional evolution are therefore revealed. Through elucidating the
mechanism of sequence, and the effects of sequence order and sequence boundary on bioclastic limestone reservoirs are
clarified. The effects of sea-level fall magnitude, exposure span and climate on formation structure, lithology and physical
properties are illustrated. Typically, the high-order sequences boundary exposes for a long time, and the stratigraphy is
weathered to a high degree. In arid climates, soilization and breccia collapse occur, and an unconformable surface can be
formed, which is not conducive to improve the physical properties of the reservoir. In humid climates, large-scale
leaching and dissolution occur, or thin carbonaceous mudstone can be formed locally. The exposure time of the low-order
sequences boundary is short, and the leaching and dissolution occurs without destroying the strata structure, which can
form high-quality reservoirs. Finally, problems and development trend in the sequence study of Cenomanian-Early
Turonian in southeastern Iraq are pointed out, providing a reference for the separated waterflooding development of thick
bioclastic limestone reservoir. |
