DISCRIMINATING FLUVIAL FROM DELTAIC CHANNELS - EXAMPLES FROM INDONESIA

摘要

The successful exploration and development of hydrocarbons from channel reservoirs requires detailed knowledge of the external geometry and internal architecture of the reservoir body. This knowledge becomes even more critical when exploring for stratigraphic traps. A classification into fluvial (meandering, braided, anastomosing), deltaic (distributary) or incised valley (fluvial or estuarine), sets strict boundary conditions that constrain the external geometry and internal architecture of the channel deposit. In the past, distributary channels have been incorrectly compared with fluvial channels. In mixed-load systems, deltaic distributary channels are frequently rectilinear channel segments located on the delta plain between the head of passes and the depositional mouthbars. Fluvial channel reservoirs are most commonly sandstone deposits of meander pointbars or braided sheets. Deltaic distributary channel reservoirs, on the other hand, are most typically elongate sandy channel sidebars attached to morphologically rectilinear channel walls. Channel sidebars form by both lateral and downstream accretion resulting from flow in a confined, low-sinuous thalweg, and may be filled with organic mud following channel abandonment. A deltaic distributary channel therefore will often show as a slightly sinuous feature on 3D seismic and can easily be mistaken for a meandering channel belt. To avoid misclassification, it is important to be conscious of relative scales. Deltaic distributary channels are usually thinner and shallower compared to their updip fluvial channel belts, and cannot be thicker than their depositional mouthbars. Width-thickness ratios for fluvial distributary channel reservoirs are on average 50:1, while meandering fluvial channel reservoirs have width-thickness ratios of typically > 100:1, and braided river reservoirs show ratios of 500 : 1 or higher.