Fluid mapping in deeply buried Ordovician paleokarst reservoirs in the Tarim Basin, western China

Abstract

The storage spaces within deeply buried Ordovician paleokarst reservoirs in the Tarim Basin are mostly secondary and characterized by strong heterogeneity and some degree of anisotropy. The types of fluids that fill the spaces within these reservoirs are of great importance for hydrocarbon exploration and exploitation. However, fluid identification from seismic data is often controversial in this area because the seismic velocity for this particular reservoir could be significantly influenced by many factors, including pore shapes, porosity, fluid types, and mineral contents. In this study, we employ the differential effective medium-Gassmann rock physics model to interpret and discuss the characteristics of conventional karstic carbonate reservoirs in the Tarim Basin that are filled with different fluids (oil, gas, and water) using logging data and thus objectively build corresponding fluid identification criteria. These criteria are subsequently evaluated by amplitude versus offset (AVO) forward analysis based on typical logging data and further applied to ascertain the reservoir fluid types in two different areas in the Tarim Basin based on prestack inversion results. For conventional carbonate reservoirs, gas can be distinguished from heavy oil and water, but heavy oil and water are broadly similar on seismic data. For condensate carbonate reservoirs, water can be differentiated from light oil (i.e., condensates) and gas, but light oil and gas demonstrate substantial similarities in terms of their seismic responses. The predicted fluid results are in good agreement with the results of drilling and oil testing. In particular, modeling the seismically resolvable reservoirs in the carbonate strata in the Tarim Basin, which have needle-and sphere-shaped storage spaces (pore aspect ratio > 0.3) and clay content that is lower than 5%, indicates that fluid properties could be properly evaluated if the porosity is larger than 5% for conventional carbonate reservoirs and >7% for condensate carbonate reservoirs.

Three-Dimensional Geophysical Characterization of Deeply Buried Paleokarst System in the Tahe Oilfield, Tarim Basin, China

“…The multi-period fractures formed by faults and unconformities are mutually cut, combined with a variety of dissolved pores, resulting in extremely strong heterogeneity [41][42][43][44][45]. The paleokarst reservoirs are the main storage space and have experienced multiple stages of tectonic movements and longtime diagenetic processes [46][47][48]. The main karstifications were in the late Caledonian and Early Hercynian, and the karst system was buried and was not reactivated as a result of epeirogenetic movements and erosion of the overburden.…”

Prestack Inversion Identification of Dolomite Reservoirs in the Fourth Member of the Sinian Dengying Formation in Moxi Area, Sichuan Basin, SW China

“…Because the velocity of P-wave decreases obviously after passing through the gas-bearing reservoir, but the velocity of Swave does not change obviously, so the velocities of P-wave and S-wave are the key parameters for reservoir and fluid identification and prediction, and the velocity of S-wave plays an important role in petrophysical analysis and AVO forward modeling [22][23][24]. The quality of S-wave curve is difficult to control, so it is necessary to effectively predict Swave velocity through petrophysical experiment and logging curve intersection analysis [25].…”

Applicability Analysis of Pre-Stack Inversion in Carbonate Karst Reservoir

“…Pre-stack inversion has rarely been applied to the Tarim Basin. A previous researcher [14] physically modeled the seismic response of a karst cave, and drew some conclusions surrounding the effects of the cave scale, velocity, spatial distribution, shape, and fluids on the "beads" and the corresponding relationships between six types of "beads" and karst caves; However, there is no validity analysis for pre-stack inversion. The authors of [15][16][17][18][19] used pre-stack inversion based on seismic data for an Ordovician TZ45 area, and indicated that pre-stack inversion can be applied for cave carbonate reservoir prediction.…”