Carbonate reservoir outcrop analogues with a glance at pore-scale (Middle Jurassic, Lusitanian Basin, Portugal)

Abstract

Abstract The Middle Jurassic carbonate successions from the Lusitanian Basin (Portugal) show a range of facies deposited across a ramp depositional system, well exposed over large areas. These facies are valuable for understanding reservoirs and related seal and trap configurations, namely in carbonate ramps. This paper documents primarily shallow-water units corresponding to the dominant reservoir-type facies, highlighting their potential as analogues, particularly at outcrop but integrating main macro and micro-scale characteristics. The mid and inner ramp sediments, which include the key studied units, consist of: peloidal-intraclastic, bioturbated, fossiliferous limestones; oolitic, skeletal, and coral-algal limestones, assigned to shoals and barrier-island complex; fossiliferous, oncoidal, fenestral deposits from lagoonal, peritidal and, locally, pedogenic settings; dolostones and dolomitic limestones. Facies with higher potential reservoir features are addressed in more detail, in three broad groups: sandbodies; biostromes; dolostones/dolomitic limestones. The remaining facies are more briefly described. The most widespread reservoir lithofacies are cross-stratified or massive oolitic and bioclastic grainstones to rudstones and coral-dominated biostromes. The larger-scale sandbody units show a dominant sheet-like geometry, but wedges and smaller-scale lensoid units occur, grading into, and interfingering with, other facies types. The reservoir facies are closely associated with, hence often encased by, flow baffle or barrier seal facies, either landwards (lagoonal/peritidal) or seawards (mid-outer ramp). This architecture, coupled with varied diagenetic paragenesis, created a range of stratigraphic and diagenetic traps that may be perceived as play similarities in potential exploration situations. The present reservoir quality is mostly low to moderate, but examples of high/excellent porosities and locally enhanced permeabilities occur. Overall, these units are highly suitable field analogues to clarify processes leading to depositional, diagenetic, fractured and hybrid reservoir types. Moreover, they clearly indicate how shoreface dynamics, shallow-water carbonate factory productivity and diagenesis lead to more complex reservoirs than usually appreciated in mostly homoclinal depositional systems.