EXPLORING THE ORGANIC/INORGANIC BORON CONNECTION IN HYDROCARBON RESERVOIRS

Studies of the geochemistry of oilfield brines have been useful in identifying trace components that can be used as indicators of the proximity of mature hydrocarbons. Many of the trace components of oilfield waters are incompatible elements, like boron, that prefer the aqueous phase, or volatile compounds (e.g. ammonium).

Our research of the N-geochemistry of organic-rich sediments showed that ammonium is released from kerogen as it becomes thermally mature, and it accumulates in brines that migrate with mature hydrocarbons. Since ammonium easily substitutes for potassium in the interlayers of illite/smectite (I/S) the sediments preserve a high concentration of ammonium where hydrocarbons have been.

Similarly, B is concentrated in oilfield brines (100’s-1000’s ppm), whereas its abundance in seawater (5 ppm) and meteoric water (<1ppm) is negligible. Past research assumed that B is released from clay rich sediments during burial, causing an increasing aqueous-B concentration with depth. However, our recent experiments showed that B-contents of I/S increase with illitization as B substitutes for Si in the tetrahedral layers of the clay lattice.

We tested the hypothesis that B is a significant component of some types of kerogen. Using an ion microprobe we measured the B-content and isotopic composition of type I, II and III kerogens and coals. If organic-B is present in kerogen it should be released during thermal maturation, similar to ammonium. The organic-B content ranged from 0-600 ppm in kerogen and up to 1370 ppm in coal. The B-isotope ratios were all negative (–10±0.4‰ to –33±1‰) showing that organic matter is a potential source of isotopically light B in thermally mature sedimentary basins. The incorporation of B in authigenic clay minerals could be useful in delineating hydrocarbon migration pathways or detecting reservoirs.