A Preliminary Study of Accumulative- pore in the Third Oil Group of Lower Xingouzui Formation in Tuoshi Area, Qianjiang Depression

This paper presents the change and the distribution of accumulative-pores during diagenesis in the Third Oil Group of Lower Xingouzui Formation in Tuoshi area, Qianjiang depression. The following are the principal viewpoints. 1. The accumulative-pores in sandstones of Third Oil Group of Lower Xingouzui formation in Tuoshi can be divided into two stages. The first stage is the occurrence of intergranular pores formed by dissolution of carbonate cements, occurred before or during the precipitation of ferrodolomite and after the precipita- tion of ferrocalcite in early- mature stage of mesodiagenesis, the porosity roughly corresponds to the porosity of sandstones that deposited before the midstage of Jinsha Formation. The second stage is the forming of intragranular pore formed by dissolution of grains, occurred after or during precipitation of anhydrite in late- mature stage of mesodiagenesis, and before overmature stage of mesodiagenesis, the porosity roughly corresponds to the whole porosity of sandstones that deposited before the third Member of Qianjiang Formation. 2. Different stage of accumulative-pores controlled the different stages of oil accumulation. The pores that formed in the first and the second stage had partly been filled by low-mature and high-mature oil respectively. However, the low-mature oil was lost and oxidized that caused by tectonic movement, so the pores formed in the first stage became unefficient pores. 3. The diagenetic evolution of sandstones controlled the change of accumulative-pors in Third oil group of Lower Xingouzui formation in Tuoshi. The precipitation of carbonate and its evolution of diagenesis as well as the dissolution of grains are the main factors that caused the two stages of accumulative-pores forming. 4. The development of accumulative-pores in sandstones are related to the sandstones' component, depositional texture, sedimentary facies and the mineralization of formation water. The sandstones that formed in storm channel are the favourite sites of acculative-pores developing, but not are the sandstones in storm interchannel and the area around well Tuo-8. 5. The resultant accumulative-pores are different in scale, quantity and effectivity of preservation because of their different diagenetic environment. The spaces that favourable for the development of accumulative-pores may also the same for hydrocarbon accumulation.