Figure 2. Caprock layers in thin-section at sample depth of 1737 ft. in the 816 well. The dark bands contain bitumen and very fine-grained calcite crystals. The bitumen is composed of heavy hydrocarbon that the bacteria could not consume. The cyclic layering of bitumen and coarse calcite is interpreted to represent cycles of bacterial growth and quiescence in the caprock accretion cycle. Thin-section photomicrograph, plane-polarized light; scale bar at lower right is 0.8mm.