Suibin Depression that covers an area of 5470 km

2

in the west part of Sanjiang Basin is a half-graben faulted depression which faulted in the west but overlapped in the east.The NE-strike faults formed in the early stage controlled strip-shape structures

while the NW-strike faults developed in the late stage cut the structures into secondary blocks that composed of one sag

three uplifts and two slopes.The depression has fine source-reservoir-cap rock assemblage and trap conditions.The two source beds of Upper Jurassic developed in marine Dongrong Formation and the nonmarine coal-measures of the Chengzhihe and Muling Formations contain abundant organic matter.Organic carbon con-tents in mudstones are 0.300%-1.675% in Dongrong Formation;0.864%-2.700% in Chengzhihe and Muling Formations

and the ratio of chloroform extract"A"to that are13.43-117.00 mg/g in Dongrong Formation;8.00-50.60 mg/g in mudstone of Chengzhiheand Muling Formations.The organic carbon contents in carbonic mudstone is 5.725%-19.081%.in Chengzhihe and Muling Formation

and the ratio of chloroform extract"A"to that is 2.0′-19.5 mg/g.According to thevitrinite reflectance R°

the Dongrong Formation is 0.67%-0.98% and the Chengzhihe and Muling Formations 0.62%-0.92%

all of them have entered into mature stage.The later two formations are mainly gas-generating rocks.Sandstone developed in Upper Jurassic occupies 50% of the total thickness.Its porosity is 1.9%-23.3%

and permeability 0.01×10

-3

-1.41×10

-3

μm

2

which satisfied the lower limit of the physical property of gas reservoir.The Upper Jurassic mudstone

being thick and laterally distributed stably

is favourable caprock.Besides

the tuff and Creta-ceous volcanic clastic rocks in local places could also be fine caprocks.The trap condition including linear anticline

diapiric structure

syndepositional anticline

lithologic pinch-outand stratigraphic overlap is good too.On the whole

the depression has favourable oil-gas forming condition

and quite a few coal-formed gas showings have been found.