The paleo reconstruction of a local stratigraphic sedimentary deposit is always a dynamic puzzle. Incised gravel or sand deposits and where in a fluvial (stream or cut bank), or marine deposit (i.e. high energy or low energy slope) the drill bit penetrates is most times the luck of the drill.
     In Medina County a well capable of irrigation (>100 GPM) might be offset 400' away and drill into a silty sand deposit (<10gpm) and miss a "high flow" gravel deposit. We perform depositional research before we drill so we can try to locate your well close to previous good wells.
     In Bexar County on the East side of San Antonio from W.W. White Rd. south to Holt and east to Dullnig-Brehm Ranch then (Eckert Fld . area)(Basalt Plug disc. 1929) then east to 1604 and New Sulphur Springs Rd. there is a very water rich meandering sand channel. On highway 87 S. @ 1604 going south towards the SE corner of Bexar into Wilson County there is only a shale sequence present and some areas here have no water wells. 
     China Grove has a few wells that all produce at different rates from 15 gpm down to 1 gpm from a sand system.
     Our dynamic Earth systems did not deposit sediments in a straight line nor incise younger formation sediments in a straight line. Incised layers are usually a higher energy deposit and can transport larger gravels (Larger porosity) rocks in these channels.