Leather thickness , when expressed in ounces means that one square foot of 16 Oz. leather weighs 16 Oz. or one pound. Further, the one ounce designation equals to 1/64" inch of thickness, therefore at 3/16 your leather is approx. 12/64 or 12 Oz. leather which is borderline too heavy for sheath work, but to each his own.
The color of your leather and the oily appearance makes me think you have a piece of Latigo which is used mostly for strap work on saddles etc. and not often used for sheaths and such. The loose fibrous flesh side of the leather (the back side) tells me the leather was cut perhaps off the belly of the hide or some other non prime area. The color of your leather is typical of most Latigo offerings, although it sometimes can be obtained in a more tan to medium brown shade also.
I think you would be a lot happier if you bought Vegetable tanned leather in about 10 Oz. weight for your sheath work. The color of your leather is about between "Chestnut" and "Brown" colors offered by Wickett &Craig Tannery. Russet or natural color veg. tan can also be used and dyed whatever color you like at the moment, but you have to buy the dye and do the work.
leather types site:wikipedia.org
leather weight designations
Some time spent at the links above might give you further guidance.
Good Luck
Paul