Geology Reference
In-Depth Information
and steep eastern flanks. Very common are crosscutting faults making
the anticlines into block structures. Numerous anticlines in the north of
the basin have mud volcanoes usually associated with large faults over the
anticlinal crests. Furthermore, there are intense neotectonic processes and
associated diapir folding due to the Oligocene clay plasticity.
Oil and gas occurrences in the Irrawaddy zone are associated with the
Middle Eocene through Lower Miocene stratigraphic interval. The pro-
duction in the Sumatra portion of the basin comes from the Pliocene
sandstones and Miocene reef limestones. With the exception of reefs, the
natural reservoirs are sheet-type and are represented by sandstone beds,
3 to 15-m thick. The number of productive intervals may reach 35 to 50.
Most of the discovered accumulations are found at depths shallower than
2,000 m.
There are certain specifics of the hydrocarbon saturation of the region:
oil and gas accumulations are spatially associated with intensely faulted
portions of the local structures; the gas-saturation of the oils increases with
depth; the section is predominantly oil-saturated; the upper gas-saturated
zone is practically nonexistent due to the faulting and mud volcanism. The
largest fields of the region are Chauk-Laniva, Yenangyang (the Irrawaddy
region) and Rantau, Arun and Gyeongdong (the Sumatra part of the basin).
1.1.5
The Los-Angeles Basin
The basin is a typical intermontane depression surrounded by the Santa
Monica and San Gabriel Mountains from the north and northeast, the
Santa-Ana Mountains from the east and Palos Verdes Hills from the south-
west. The basin's tectonic boundaries are large deep-seated regional faults
at the feet of these mountains. In the north, it is the Santa Monica and San
Gabriel fault system, in the east the Elsinore-Chino fault system, in the
south Christianitos Fault, and in the west Palos Verdes Fault (Figure 1.7).
The section is represented by Upper Cretaceous through Pleistocene
rocks overlying the Jurassic basement. The western part of the basement is
most uplifted. There, the Cretaceous and Paleogene are missing from the
section and the crystalline basement is overlain by Miocene sediments.
Average thickness of the sediment cover in the basin is 6 km; the sec-
tion's clay content is 46%. The Upper Cretaceous is composed mostly of
arkose sandstones overlying the basal conglomerates with total maximum
thickness of up to 2,700 m. The Paleogene formation comprises argillites,
variegated clays, sandstones and in conglomerates of the upper section
(total thickness of up to 4.600 m); the Miocene complex is composed of
the sequences of alternating red-bed clays and sandstones, up to 3,500 m
