Evaluating Hydrocarbon Expulsion Efficiency from Shale Reservoirs

Arango, Irene
[email protected]

One of the main aspects that define a successful shale gas play is the presence of enhanced production intervals or sweet spots. Sweet spots are intervals within the shale reservoir that contain elevated quantities of natural gas. Successful pre-drill prediction of the location of the sweet spots in a given play requires understanding of the geologic processes conducive towards the concentration of economically-relevant gas volumes in the unit of interest. Of particular importance is the understanding of the kinetics of gas generation: Gas generation within a given source rock requires adequate amounts of organic matter and enough time for the thermal breakdown or "cracking" of the organic matter to take place (maturation). Although kerogen can be transformed directly to gas (primary cracking), cracking of liquid hydrocarbons to gas (secondary cracking of oils or condensates) appears to be a process of significance in shale gas systems, requiring low liquid hydrocarbon expulsion efficiencies.

In the case of type II kerogen-rich rocks (e.g., most shale gas units), the initial products of maturation are liquids. With increasing maturity, higher amounts of liquids within the shale pores results in increasing pressure, driving fluid expulsion out of the rock and into the surrounding carrier system. The expulsion efficiency of liquids from a source rock is facilitated by the availability of and proximity to porous media (Leythaeuser et al., 1988; Lafargue et al., 1994). In traditional systems, source rocks are generally considered to have high expulsion efficiencies. However, shale gas systems are considered self-contained (both a source and a reservoir); therefore, expected to behave less efficiently from an expulsion stand-point. Following on the aforementioned rationale, a shale-gas unit is expected to retain more of the liquids generated.

As proximity to porous media favors expulsion, the total thickness of the unit is expected to play a major role in the overall retention of liquids and subsequent amounts of gas generated. Larger concentrations of hydrocarbons should occur towards the center of thick shale reservoirs, while expulsion should increase towards the bounding porous units. This study focuses on the evaluation of the impact of total thickness on expulsion efficiency and how this parameter may influence the amounts of gas generated and retained within shale gas plays.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013