Inefficient Charge-Migration or Poorly Characterized Traps

Abstract

Considerable effort is applied to model charge efficiency in the hydrocarbon exploration process. We suggest that in most cases, it can be shown that structural traps are either completely filled (filled down to the shallowest leak point), or they are not filled at all. There are actually very few partially filled traps.Recently the AAPG nominated U.S. Allan’s 1989 paper on fault juxtaposition analysis as one of the ten most important papers in oil and gas structural geology. The paper proposed that across fault juxtaposition is the key control on structural/faulted traps. Since this work has been published a significant number of studies have simply asserted that fault membrane sealing occurs, even though comprehensive evaluation of fault juxtaposition or other alternatives which could explain the observations has not been completed.We note three other potentially important considerations for charge risking analysis. First, fault membrane seal calculations should always be combined with juxtaposition analysis, which implies that membrane seal results should always be presented with Allan maps. Secondly, without exception, fault membrane seal calculations will provide more optimistic estimates of hydrocarbon height than juxtaposition alone. Third, we note that many recent studies use a work flow that has questionable application to charge risk, namely they back calculate the pressure capacity by using a Shale Gouge Ratio (SGR) or similar algorithm, and then arrive at a trap capacity and column height prediction. Importantly, this work flow is almost always conducted using single “best” technical models with no direct modelling of uncertainty. Along strike or spatial variability of fault rock properties are also very significant. A series of faults in Miri, Sarawak, have been systematically mapped in great detail to measure the strike variability of fault rocks. This work aids understanding of the commonly noted problem of “holes” in fault membranes. Results of a significant number of hydrocarbon traps will be presented in which observed hydrocarbon water contacts are compared with probabilistic predictions of hydrocarbon column heights and trap capacity. Models which consider juxtaposition alone are more accurate, and as such, these should be used in future analyses of charge efficiency.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019