Perched-water conditions have been observed infrequently in oil and gas fields in zones above fine-grained material that may be located a few to 100 meters above the oil water contacts. Although infrequent in scale of number of fields globally, perched-water zones are important consideration when evaluating reserves and planning field development. The study described in this paper was conducted to examine perched-water conditions and quantitatively evaluate factors that control perching behavior and its impact on field development plans.
The differences between perched-water and hydrodynamically-tilted contacts have to be understood and clearly identified in a given reservoir by studying geological controls. The geological controls that shape contacts either perched or hydrodynamically tilted do play vital role in field development plans because of their structural differences leading to pressure communication or the lack of it. A perched-water zone in subsurface is defined as a water-saturated zone that is above contact or as a water-saturated zone not-directly-connected to the regional oil-water contact. Perching phenomena may occur in a permeable layer overlaying a relatively impermeable layer and conditions that could force water to infiltrate vertically through low permeability layer.
The stress conditions that lead to formation of perched-water are extraneous. However, petrophysical properties of the subject rock that is exposed to the prevalent conditions determine the viability of formation of a perched-water structure. Hence, neither structural geology, stratigraphy and depositional environment studies nor facies and petrophysical evaluations can be performed alone to decipher if a structure is/can be considered "perched-water". Therefore, the cases evaluated in this study are interrogated from structural, hydrodynamic and petrophysical perspectives to decide if subject structures can be deemed to contain "perched-water". The correct diagnostic brings additional hydrocarbon pore volumes or cost savings when drilling below apparent hydrocarbon-water contacts with misconception of a probable perched-water both affecting the financial positively