Page 371 - Petroleum Geology

341

steeper flank tends to be that on the seaward side, away from the source of
the regression. The main petroleum accumulations are in the gentler anticlines.

Gesa anticline, Waropen coast, Irian Jaya

In the North Coast basin of Irian Jaya there is a very young regressive se-
quence under the Mamberamo delta (Fig. 15-12). The basin lies south of the
present coast-line, where basement was found by drilling and geophysics at
about 1200 m, rising to the north. A line of mud volcanoes occurs in the
swampy plains of the Mamberamo delta. This line, about 35 km long (22
miles), forms a part of a general line about 100 km long (60 miles). The area
has been described by Visser and Hermes (1962). Some of the mud volcanoes
are on a narrow ridge, with outcrops of Plio-Pleistocene sandy and argillaceous
rocks. Warm salt water and some methane are extruded with the mud. Further
south, inland, there are some anticlines in which Plio-Pleistocene paralic sands
with lignite beds and subordinate soft muds are exposed. The anticlines to
the south tend to be steeper, with dips nearly vertical in their cores; but the
first one to the south, the Gesa anticline, is a very gentle elongated dome
with a sinuous axis roughly parallel (as are the others) to the line of mud vol-
canoes. Into this anticline, two boreholes were drilled between 1956 and

Fig. 15-12. Map of part of the Mamberamo delta, Irian Jaya. Mud volcanoes shown with
diagonal lines. Relevant Plio-Pleistocene outcrop stippled. The Rombebai lake to the east
of the line is probably evidence of local subsidence. (After Visser and Hermes, 1962, en-
closure 1-111.)