Complex sentence: Tight oil shale formations are heterogeneous, they vary widely over relatively short distances, thus even in a single horizontal drill hole the amount recovered may vary, as may recovery within a field, or even between adjacent wells.
Split into simpler sentences: Tight oil shale formations are heterogeneous and vary widely over relatively short distances. Thus, even in a single horizontal drill hole, the amount recovered may vary, as may recovery within a field or even between adjacent wells.