Abstract

In exploration wells, wireline geophysical well logs and drilling cuttings are the basic data which indicate whether a trap has been found. In cases where boreholes are in poor shape or drilling methodology results in low quality drill cuttings the well logs are often relied on to define the presence and quality of a trap. However, the standard well log data and the common methods of log interpretation may cause a firm not to test a well and to walk away from a subtle trap comprised of a heterogeneous reservoir rock.

There are a number of types of heterogeneities. Ail reservoirs are heterogeneous at some scale or in some characteristic. However, the common types of heterogeneities which might cause a subtle trap to be missed are generally related to either the determination of porosity or fluid content using well logs. Although large-scale heterogeneities exist and can cause the misevaluation of the potential of reservoir, this paper addresses the problems of recognizing and correctly interpreting decimeter-to-micron size heterogeneities. Heterogeneities smaller than half a meter can cause the log interpretation of lithology, porosity, and oil saturation to be significantly in error because of the ½ meter or larger vertical resolution of well logs. Standard interpretation models do not include all of the types of reservoir heterogeneity.

There are a variety of decimeter-scale reservoir heterogeneities which can commonly occur: thin beds of either sandstone or carbonate interbedded in a shale sequence are ubiquitous in many areas. Trough-filling shales of limited areal extent may be incorrectly interpreted as shale beds which continue away from the exploration well with the same thickness. Noduies of dense materia) within a reservoir zone, such as anhydrite in carbonate and clay galls or siderite in sandstone, also can cause the trap to be pessimistically evaluated. The composition of immature sandstones and heavy-mineral bearing sandstones can likewise result in a trap being missed. Thin porous beds within either tight sandstones or carbonates are generally incorrectly interpreted or totally missed in exploration wells.

Although there need be no misinterpretation of reservoir traps with the present technology of the petroleum industry, the challenge is to know what intervals in an exploration well need to be evaluated in a more thorough manner. Many heterogeneities are difficult or impossible to define using standard well log data or routine computer interpretation programs. However, there are ways to reveal the probable presence of reservoir heterogeneity using drill cuttings and the well logs generally available for an exploration well. Furthermore, traps with each of the above types of heterogeneities have been correctly evaluated with a careful integration of data - well logs, drill cuttings, cores from other wells, drillstem tests, and especially the geological interpretation of all available data.