TITLE:
Characteristics of an Upper Jurassic Carbonate Ramp in the Northern Amu-Darya Basin
AUTHORS:
Yongyao He, Han Mu, Yuanyuan Kang, Yingmin Wang, Bojiang Fan
KEYWORDS:
Carbonate Ramp, Facies Associations, Sequence, Amu-Darya Basin
JOURNAL NAME:
International Journal of Geosciences,
Vol.9 No.2,
February
28,
2018
ABSTRACT: Overemphasizing the value of reefs in oil and gas exploration, reef-oriented
geologists explain all carbonate platform deposits using the Wilson model. In
their eyes, rimmed shelves are more valuable than carbonate ramps. However,
organic banks are excellent reservoirs generated by carbonate ramps in the
study area, as verified beyond doubt through petroleum exploration, such as
this thesis, which investigates the genesis, types, and distribution of carbonate
deposition in the north zone of the Amu-Darya Basin. Monoclinal palaeogeomorphology
and rudists suggest shallow environments. Given that oolite
shoals and rudist patch reefs were observed in the study area, the depositional
system is interpreted to be a carbonate ramp. The Callovian-Oxfordian stage
consists of nine lithofacies: oolitic limestone, skeletal limestone, micritic limestone,
bioturbated limestone, and crystalline limestone, which are grouped
into three facies associations presenting outer ramp, mid-ramp, and inner
ramp facies associations. Five depositional sequences can be distinguished in
the Callovian-Oxfordian stage. Each third-order depositional sequence is
composed of transgressive systems tracts (TST) and highstand systems tracts
(HST). The TST consists of mudstones with a higher response to natural
gamma rays, whereas the HST contains various types of grainstone, with subordinate
dolostone. The vertical and lateral distributions of sedimentary facies,
and their interpreted depositional environments, revealed a ramp exhibiting
a gradual southeast-northwestward environmental change from outer
ramp, mid ramp, and inner ramp carbonate facies.