Understanding reservoir performance and predicting hydrocarbon recovery in carbonate reservoirs are challenging due to the complexity of the pore system and the dynamic interplay of multiphase fluids that move through the pore network. A multiyear study of carbonate reservoirs across a broad spectrum of geologic conditions, fluid types, and field maturities has resulted in key insights on the links between pore-system characteristics and dynamic fluid-flow behavior with material relevance to carbonate resource assessment, field development optimization, and maximizing ultimate recovery. Pore-system heterogeneity is a primary control on hydrocarbon displacement efficiency. Multiphase flow through heterogeneous pore systems with a mix of pore types results in lower recovery than flow through more homogeneous pore systems. Due to the homogeneous nature of the micropore system, rocks dominated by micropores can have favorable hydrocarbon displacement with residual oil saturation to water displacement (Sorw) less than 5%. Rocks with a heterogeneous mix of interparticle and micropores have less favorable displacement, with Sorw as high as 20%, despite having higher permeability. A threshold of approximately 80% microporosity appears to distinguish: (1) more favorable displacement in micropore-dominated rocks vs. less favorable displacement in rocks with a mixed pore system, (2) the magnitude of permeability for a given porosity in mixed vs. micropore systems, and (3) the proportion of microporosity above which pore space of any type is connected exclusively through the micropore network and flow properties reflect the homogeneous nature of that pore system. Within the homogeneous micropore system, Sorw increases from about 5% to 20% as porosity and permeability decrease and micropore type transitions from type 1 (higher quality) to type 2 (lower quality). A major control on multiphase fluid movement in reservoirs with interlayered mixed and micropore-dominated flow units is the contrast in capillary pressure (Pc) and water relative permeability (Krw) between these distinct pore systems. When compared on a consistent basis, 60% water saturation, for instance, rocks with a mixed pore system have approximately neutral (0 psi, 0 kPa) Pc values and higher Krw values, whereas rocks dominated by microporosity have more strongly negative (–6 psi, –41 kPa) Pc values and lower Krw values. In the case of a water flood operation, this contrast in Pc and Krw can lead to more heterogeneous sweep patterns and lower recovery. A new method for tagging in-place oil with xenon was coupled with flow-through micro-computed tomography imaging to directly investigate oil displacement under water flood conditions. The results provide a qualitative demonstration of how brine flooding displaces xenon-saturated oil. Displacement patterns in micropore-dominated rocks are homogeneous and compact with limited bypass of oil, consistent with relatively low Sorw. Conversely, the displacement pattern in rocks with a mixed pore system is more heterogeneous and exhibits significant regions of bypassed oil, consistent with higher Sorw and Krw.

Abstract The Nanpanjiang Basin (NPJB) is a large, complex basin within the south China plate bordered by Precambrian uplifts on the northeast, southeast, and west and by a Triassic suture zone to the south. During the Permian and Triassic, the NPJB formed an embayment in the Yangtze Platform (YP) and contained several isolated carbonate platforms (IPs), including the Great Bank of Guizhou (GBG) and the Chongzuo–Pinnguo Platform. The NPJB presents an exceptional natural laboratory for evaluating controls on carbonate platform margin and slope architecture. Multiple twodimensional transects through the YP and IPs provide exposure along spatial and temporal gradients in tectonic subsidence rate, siliciclastic input, antecedent topography, and oceanography. Platform development across the end-Permian extinction and evolving seawater chemistry allow assessment of the impact of carbonate factory change from a basin-wide perspective.The YP and IPs evolved from ramps and low-relief banks with oolite margins and mud-rich slopes early in the Early Triassic to steepening Tubiphytes-reef rimmed platforms with slopes progressively enriched in clast-supported breccias in the Middle Triassic. Despite differences in the slope angle and windward–leeward differences in grain size at the bank margin, the Early Triassic margin-slope systems have very similar characteristics throughout the basin. During the Middle Triassic, the YP and IPs developed extreme lateral variability in margin architecture due to differences in tectonic subsidence and siliciclastic basin fill at the toe-of-slope. The southwestern sector of the YP and the GBG drowned under pelagic carbonates followed by siliciclastic turbidites in the Late Triassic, Carnian, while the northeastern YP continued shallow-marine deposition until burial by prograding shallow-marine siliciclastics. The southerly IPs have backstepping geometries, terminal pinnacles, and earlier drowning and burial by siliciclastics. Differences in antecedent topography affected margin width and stability, resulting in changes from broad aggrading to prograding margins vs. high-relief and collapsed margins. Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, backstepping geometries, and pinnacle development. Timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and progradation at basin margins. Development of ramp profiles with oolite margins in the Early Triassic and subsequent development of steep-sided margins in the Middle Triassic reflects changes in carbonate factory type following the end-Permian extinction. Process-based depositional models derived from the NPJB can aid in the prediction of facies distribution and architectural styles at the basin scale in other systems, particularly in areas of active tectonism and temporal variations in oceanographic conditions, such as, for example, in the prolific Tertiary carbonates reservoir province of southeast Asia.

Abstract Comparative analysis of platform evolution recorded along multiple 2D platform-to-basin transects of the Triassic Yangtze carbonate shelf and several isolated platforms in the Triassic Nanpanjiang basin, south China, indicates that laterally variable tectonic subsidence, rate of basinal clastic deposition at the toe of slope, antecedent topography, and changes of carbonate factory type controlled the evolution, large-scale sequence stratigraphic architecture, and geometry of the platform margin and slope. Lateral and temporal changes in these parameters, and their various combinations during the Middle and early Late Triassic, were responsible for the remarkable vertical and along-strike variability in the observed platform architecture and slope profile. Timing and rates of subsidence largely controlled along-strike variability, timing of drowning, back-step geometries, and pinnacle development. Antecedent topography and timing of clastic basin fill dictated differences in platform-margin stability and geometries such as slope angle, relief above basin floor, development of collapse scars, and progradation at basin margins. Changes in slope profile through the Early and Middle Triassic reflect changes in carbonate-factory type and evolving seawater chemistry following the end-Permian extinction. Eustasy, in contrast, had very little influence on platform morphology and large-scale architecture. Process-based depositional models derived from the Nanpanjiang basin of south China present an important analog for understanding, quantifying, and predicting facies distribution and architectural styles at the basin scale in other systems, particularly in areas of active tectonism and temporal variations in oceanic conditions, such as, for example, the prolific Tertiary carbonates reservoir province of southeast Asia.