Evidences and Mechanism for Hydrothermal Dolomite in the Lower Paleozoic in Tarm Basin

There are some light-gray or white coarse crystalline saddle dolomites saddle dolomites in the Cambrian and lower Ordovician brown-gray very fine-fine crystalline matrix dolomites in Tarim basin. The saddle dolomites include two types, one filling in vugs or fractures (FD) and the other occurring as patches (PD) near fractures in matrix dolomites. Both FD and PD have curved crystal face and show wave-extinction under cross-polarized light. Most of homogenization temperatures of fluid inclusions in saddle dolomites are between 120° and 160°, higher than present stratum temperature. The salinity of fluid inclusions is above 10% Nacl.Eq., and the maximal is up to 19.7% Nacl.Eq.. Compared with matrix dolomites, the saddle dolomites have relatively low Na, K and Sr but relatively high Fe, Mn and Ba contents. The δ18OPDB values of the saddle dolomites in Cambrian and Ordovician are between -10.29‰ and -4.04‰ and -7.53‰ in average, and they are lower than those of the Cambrian and Ordovician matrix dolomites. The 87Sr/86Sr values of the saddle dolomites are between 0.709065 and 0.714090 and 0.710368 in average, and they are higher than those of the matrix dolomites.

The evidences from petrology and geochemistry indicate that both FD and PD are hydrothermal dolomites. In general, the formation mechanism for hydrothermal dolomite is thought to be that limestone is replaced by dolomite under effects of hydrothermal activity. However, the hydrothermal dolomites found in Tarim basin are not replacement-dolomite. The FD were precipitated directly from hydrothermal, and the PD were product of recrystallization of matrix dolomite under effects of hydrothermal activity. The hydrothermal in Tarim basin was related to the intensive and extensive magmatic-volcanic activity in the Permian. During the formation of FD and PD, the hydrothermal altered and dissolved the matrix dolomites. And therefore, a large number of secondary pores, such as inter-crystalline pores and dissolution pores, were produced in the dolomites. The development of the secondary pores are extremely significant for the Cambrian and lower Ordovician dolomites to evolve into high-quality reservoir beds.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California