Spectrum of pore types and networks in the deep Cambrian to Lower Ordovician dolostones in Tarim Basin, China

The complex tectonic activities and multi-stage diagenetic modifications lead to the formation of a heterogeneous pore network spectrum ranging from macropores (centimeter-size) to nanometer-size pores (< 1 mu m) in the deep Cambrian to Lower Ordovician dolostones in Tarim Basin, China. Thin section and Computed Tomography (CT) scanning were performed to obtain the petrography of dolomites, 2-D images of wide variety of pores and 3-D representation of the pore networks. Nuclear Magnetic Resonance (NMR) transverse relaxation time (T-2) distributions at saturated and centrifugal conditions were performed to obtain the continuous pore size distribution, and derive NMR parameters including Bulk Volume of Immovable fluid (BVI), Free Fluid Index (FFI), T-2gm (T-2 weighted mean on a logarithmic scale), and T-2cutoff (transition T-2 value separating BVI from FFI). Three types of matrix dolomites (dolomicrite, microbial dolomite and crystalline dolomites) and two types of dolomite cements (saddle dolomite or crystalline dolomites) are recognized in the Cambrian to Lower Ordovician dolostones. The pore systems include open vugs (enlarged dissolution pores), fabric dissolution pores, intercrystal (dissolution) pores, micro-fractures, as well as intracrystal pores, forming dual or multiple pore networks. The NMR T-2 spectra are mainly characterized by bi-modal distributions and mostly right-skewed. The very low T-2 components (< 50 ms) are associated with the intracrystalline pores with small pore sizes (< 10 mu m). The left peaks with T-2 components ranging from 50 ms to 500 ms relate to the intercrystal and intercrystal dissolution pores with pore sizes ranging from several microns to about 100 mu m. The right peaks (T-2 > 1000 ms) represent the vuggy pores with pore sizes spanning from hundreds of mu m to thousands of microns. The transition zones (100 ms < T-2 components < 1000 ms) referred to the fabric dissolution pores with pore sizes spanning from tens of microns to about 200-300 mu m. The micro-fractures have tail T-2 distributions. The various pore types are identified in the whole-diameter core CT scanning and FMI (Fullbore Formation Microlmager) image logs. Fluorescent thin sections confirm the favorable oil-bearing potential in intercrystal and intercrystal dissolution pores as well as fabric dissolution pores and micro-fractures. The continuous distribution of pore networks in dolostones can be fully characterized by using multiple-approaches. This study highlights the challenge to describe the various pore types covering several orders of magnitude in size in the heterogeneous dolostones, and will support the hydrocarbon recovery enhancing in dolostones worldwide.