Characterization of Dolomitized Carbonate Reservoirs in Lower-Middle Ordovician Yingshan Formation, Northwestern Tazhong Area, Tarim Basin, NW China

Qingyu Huang¹, Shaonan Zhang², Hairuo Qing³, and Siyang Zhang³
¹College of Energy Resource and State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China
²State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China
³University of Regina, Regina, SK, Canada

The Lower-Middle Ordovician Yingshan Formation in central Tarim Basin, NW China, is dolomitized to varying degrees in the lower 200m~300m of its 450m-thick shallowwater platform carbonate succession. The intercrystalline porosity, dissolution vugs and fractures were preferentially developed in the dolostone intervals. Understanding the processes of dolomitization is, therefore, crucial for the prediction of the connectivity and spatial distribution of dolomite reservoir.

Three petrographic types of dolomite, including two types of replacive matrix dolomite and a void-filling dolomite cement, were identified based on fabrics and textures: Very fine to fine crystalline (50-250μm), planar-e (s) matrix dolomite (MDI) is light grey in hand specimen with porous sucrosic texture and partly preserved precursor lithologic texture. It also occurs locally in the dolomitized burrow infills. Microscopically, it consists of euhedral to subhedral dolomite crystals with the planar/regular intercrystalline boundaries. Some crystals have a cloudy center and clear rims toward the pore space. The reservoir of MDI is characterized by intercrystalline and vuggy pores. MDI dolomite has δ¹⁸O values between -6.0 to -9.1‰PDB (mean -7.04‰PDB), δ¹³C values between -0.1 to -1.9‰PDB (mean -0.76‰PDB) similar to those of the Middle-Early Ordovician marine calcite (δ¹⁸O and δ¹³C values ranging from-6.5 to -8.5‰PDB, -0.5 to -1.5‰PDB, respectively), and ⁸⁷Sr/⁸⁶Sr ratios from 0.708021 to 0.708526 (mean 0.708301) that mostly overlap with the coeval seawater value (0.70790-0.70840). These features suggest that MDI probably formed from reflux of Ordovician slightly evaporitic (penesaline) seawater that did not reach the salinity required for gypsum precipitation during early diagenetic stage.

Fine to medium crystalline (100-500μm), nonplanar-a matrix dolomite (MDII) is medium to dark grey in cores with two kinds of fabrics: one occurs dolomite patches along the stylolites, the other is the massive or laminated dolomite with precursor texture completely destroyed. Microscopically, MDII is composed of subhedral to anhedral crystals with nonplanar/irregular intercrystalline boundaries and weak undulatory extinction. This dolomite lacks intercrystalline porosity, but dissolution vugs and fractures are locally developed and filled by minor amount of late-stage saddle dolomite cements (SDC) that followed by sparry calcite cements. Other authigenic minerals identified in MDII include fluorite, barite and quartz. Some MDII have δ¹⁸O and δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios similar to those of the coeval seawater, which indicates that ambient seawater was again a likely dolomitizing fluid; whereas others have more negative δ¹⁸O values (-7.2 to -10.7‰, mean - 9.41‰PDB) and the wilder range of ⁸⁷Sr/⁸⁶Sr ratios (0.707193 to 0.714287, mean 0.708552), suggesting that: 1) dolomitization may have occurred at higher temperatures during shallow to intermediate burial conditions; 2) Dolomitizing fluids might be related to the seawater mixed with unradiogenic, mantle/hydrothermal fluids associated with volcanic activities, based on the factors that more fractures and vugs were developed in these MDII with lower ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values and filled by authigenic hydrothermal minerals, such as fluorite and barite.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013