(1) China National Petroleum Corporation, Beijing, China
(2) Qihai Oil Company and Northwest University
(3) Zhejiang University, China
Abstract: Vaterite and portlandite in the debris of oil-field drilling
Vaterite and portlandite are not common in sediments, but we find these two minerals by X-ray analysis from the samples of marine carbonate rock. We take the samples at burial depth of 3500m within an over-pressured gas reservoir in Kuqa foreland-basin, west China. Vaterite, a CaCO3 polymorph, is an unstable mineral of carbonate that is said to be easily transform to calcite or aragonite in a little higher temperature than normal conditions. Our former studies have just encountered this mineral in the boreholes at south China sea basin, and some vaterite was sampling from the marine bottom sediment of south China sea. Others (Friedman, 1994) indicated that vaterite is a precipitation during drilling which suggest the nature gas dissolved original carbonate. In our studies, the vaterite is a crystalline in the debris samples and not in the form of a colloid material. Usually, the portlandite is found in cements and may be found in Tertiary igneous rock (by Roselle, 1995), but not much in sediments. Our experiments (repeated) show the portlandite mineral clearly in the debris. We have a clear XRD-Pattern which show an abundance of the two minerals and less of calcite. This discovery may be the first report of sediment basin. Combined other geological data and methods, we concluded that the over-pressured compartment is the key to the two minerals presence and it preserved in the sediments at subsurface of the petroliferous basin.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana