| Abstract |
Three types of fluids reside inside the Sandawa Collapse, namely, the acid NaCI(+SO,), the neutral-pH, NaCl brine and the acid SO4-Cl fluids. The first type of fluids occupies the central sector of Sandawa Collapse marked by wells KN-ID, KN-2D and KN-3. It contains high amounts of dissolved SO, and HSO, both of which have products of sulfuric acid dissociation, and its major dissolved salt is NaCI. At reservoir conditions, sulfuric acid dissociates to HSO4, and H+ with only a mole H+ liberated, the fluids have an apparent neutral reservoir pH. The reservoir fluids are in-equilibrium with K-mica, and this is confirmed by the presence of abundant secondary biotite in the rock cuttings. Upon boiling to lower temperature, sulfuric acid dissociates to SO, and 2H thus shifting the fluid pH to acidic level. The fluids containing more than 500 mgdg SO, (KV-1Dj became in-equilibrium with kaolinite upon boiling, but those having less than 500 mgkg SO, remained in equilibrium with K-mica. Neutralization processes through deposition of sulfur-bearing minerals like pyrite, chalcopyrite, bornite and anhydrite effectively cleanse the acid NaCI (+SOJ fluids of dissolved sulfur species. From originally acid fluids, neutralization processes transform the acid NaCI (+SOJ fluids in Sandawa Collapse to the second type or the neutral-pH NaCl brine in Marbel Corridor. The fluids in Wells KV-3 and TM-2D typify the transition between the two fluids, in which SO4res, is between 80-150 mgkg and pH lab is above 4.50. The second type of fluids have SO, less than 100 mgkg; its dominant dissolved sulfur species is H2C03 is the chemical species controlling the acidity of the fluids. Upon boiling, H2C03 is consumed by the escape of COzg, thus making the fluid pH at near-alkaline level. Both the reservoir fluids are equilibrated with illite, pyrite and chalcopyrite and saturated with calcite. The third type of fluids occurs around the Mt. Apo peak. It contains high dissolved SO4, HSO, and HC03, low Cl and high gas contents. The fluids are akin to the shallow-formed acids of Bac-Man on the basis of low CI contents and excess Na+K in the solution. It is therefore postulated that the acid condensates on the shallow level of Mt. Apo peak is presently descending to the intermediate depths. The discharge fluids from Well TO-ID typify the third type of fluids. |