Applying HC Fingerprinting Technology to Determine the Amount of Oil Produced from Hydraulically-Fractured Wolfcamp Reservoirs Using Petroleum Samples Extracted From Conventional Core Plugs

Abstract

It is difficult to obtain the end-members required to allocate commingled oil samples produced from a shale reservoir that has been hydraulically fractured – and then use those results to determine from which zones oil is actually being produced. We demonstrate that suitable end-member oils can be extracted from core plugs selected from a conventional core collected from a mudrock interval in the Wolfcamp Formation using water-based mud that consists of good oil-prone source-rock beds. End-member oils also can be extracted from most core plugs selected from an overlying conventional reservoir that contains oil expelled by the source-rock beds in the shale reservoir. Several standard geochemical source and maturity parameters confirm the results of a Hierarchal Cluster Analysis utilizing 24 HRGC peak-height ratios that shows the shale reservoir can be divided into five distinct zones that each contains a different type of oil generated at a different level of maturity by the kerogen organic facies present in that zone. HC fingerprinting results using >100 HRGC peak heights indicate that oil samples produced from two horizontal wells drilled near a stratigraphic boundary in the shale reservoir principally produce the kind of oil present in a distinct ≈40-foot zone immediately above those horizontal wellbores. An oil sample produced from a vertical well completed in the conventional reservoir produces a mixture of the type of oil extracted from a core plug selected in the lower part of that reservoir – plus the type of oil present in the same zone in the deeper shale reservoir that also accounts for most of the oil produced from the horizontal wells. We conclude that a significant amount of oil is not being produced from other zones in the shale reservoir, or from the upper part of the conventional reservoir. Implementing this technique has the potential to improve the effectiveness of hydraulic fracture programs by identifying the specific reservoir intervals from which oil is produced after a horizontal well is completed, and it increases the value of legacy and new conventional cores collected from shale-oil reservoirs using water-based mud.