Dongfang 1-1 gas field in the mud diapir belt of the Yinggehai Basin, South China Sea

Abstract

The Dongfang 1-1 (DF1-1) mud diapir field is the largest gas field discovered in the Yinggehai Basin up to now, with a proven gas reserve of 99.68×10 9 m 3. The gas pay zones are within the Pliocene fine sandstones and siltstones of the littoral and sand bar facies, with normal pressure regimes. The gases include variable amounts of CO 2 and N 2 in addition to hydrocarbon gases. The hydrocarbon gases (with the exception of biogenic methane) and nitrogen originated from the marine source rocks within the overpressured Lower to Middle Miocene strata, while CO 2 is most likely the product of thermal decomposition of the Tertiary calcareous mudstones and pre-Tertiary carbonates through relatively long-distance vertical migration. The field has received at least four phases of natural gas charging occurring relatively late, most likely during the post-Pliocene Himalayan orogeny. Diapiric faults act as the main conduit for upward movement of fluids released from the deep high-pressure system, with abnormally high pressures providing the significant driving force for gas migration. Because of the episodic nature of the deep high-pressured fluid release and the diapir fault activity, this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation.