Defining Unconventional Sweet Spots

Abstract

A simple classification of unconventional reservoirs is useful in predicting the risk associated with defining sweet spots by mapping areas of high initial production or forecasted EUR. The classification system distinguishes reservoirs based on hydrocarbon phase and whether they have in situ or migrated hydrocarbons.

Unconventional dry gas reservoirs are driven by gas expansion and behave predictably as the reservoir pressure declines. In unconventional wet gas reservoirs, the gas is driven initially by expansion. If the reservoir pressure drops to the dew point, gas condenses to liquid condensate which may block flow. In unconventional volatile oil reservoirs, oil is initially produced by expansion of the reservoir fluids; as the pressure falls below the bubble point, gas comes out of solution, and production is driven by expansion of the gas phase. At this point, the GOR increases dramatically, because the relative permeability of gas is much greater than that of oil, resulting in greatly diminished oil production.

Unconventional shale reservoirs contain in situ generated hydrocarbons. These reservoirs act as closed systems, and the volume difference created by the maturation of kerogen to oil and the cracking of oil to gas creates overpressure in the reservoir. Reservoir pressure has a major impact on in-place hydrocarbon volumes and recovery, especially in the case of wet gas and volatile oil reservoirs, where initially overpressured conditions can slow the onset of bubble point or dew point. The distribution of hydrocarbon in stacked shale reservoirs should agree with regionally mapped thermal maturity windows. Thus, if the sweet spot of a shale play is associated with a specific GOR, it can be defined by mapping thermal maturity from vitrinite reflectance, thermal alteration index or geohistory modeling.

In contrast, hydrocarbons produced from tight, non-shale reservoirs were not generated in situ, but migrated into those rocks. Understanding the source and migration pathway is crucial in these plays, because the sweet spot may not agree with regionally mapped maturity trends. Furthermore, because hydrocarbons migrate from areas of high pressure to areas of low pressure, tight non-shale reservoirs are frequently normally pressured, and may reach bubble point or dew point earlier in their production history than shale reservoirs with similar GOR.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018