Sedimentology of Low-Permeability Clastic Gas Reservoirs: A Case from the Lower Tertiary, Dongpu Depression

The gas formation of the lower Tertiary in the Dongpu Depression has been buried to a depth of over 3,500 m, and the gas reservoirs are low both in porosity and in permeability, Litkologic and lithofacies variations have played a controlling part in the reservoir's occurrence. The major gas-bearing belts are fanlike sandstone bodies in fan deltas and sublacustrine fans and stringlike sands in gravity flow channels, in which both porosity and permeability decreased with increasing compaction and cementation during burial diagenesis. Heterogeneous dissolution has taken place in the deep formations, and as a result two to three secondary porosity belts were developed at great depths: 2,600-3,000, 3,700-3,800, and 3,950-4,150 and even up to 5,600 m.