Geological Characteristics of Lacustrine Tight Oil Sweet Spots in China

Denghua Li and Guosheng Zhang
PetroChina

Since horizontal wells with multi-stage fracturing provide better economics for lower permeability reservoirs,many lacustrine tight oil plays have been discovered in main petroliferous basins in China for the past few years,such as in Ordos, Junggar and Songliao basins.Currently,many horizontal wells have been drilled,and tight oil exploration in China shows promise.

Four tight oil play types distinguished include the shale oil plays that the reservoir is also the source rock, such as the Shahejie shale in Jiyang depression of Bohai bay basin;the sandwich type plays which reservoirs are interbedded with shales,such as the Chang 7 Formation in Ordos basin;the hybrid plays which reservoirs are mixed with organic matters, such as the Lucaogou Formation in Jimusaer Sag of Junggar Basin ;and adjacent source plays that the reservoir is just on the top of the source rock or at the bottom of it,such as the Fuyu Formation in Songliao basin.Each play is comprised of the lacustrine shale source rock with different low permeability reservoirs consisting of dolo silts,siltstones,carbonates,tuffaceous mudstone or shales.

All types of tight oil plays, except shale oil ones ,have gotten commercial output,and sweet spots are controlled by three major factors.The first one is large volume of superior source rock,which supplys ample petroleum;the second one is high porosity zone,while high production wells are usually located in the area porosity more than 6%;the last one is nature fractures rich zone,while opened fractures and large faults are damaged to plays,only sealed fractures are advantageous to fracturing and able to improve the permeability,such as Da’anzhai Formation in Sichuan basin.

Finally, the comparison of Sino-U.S. tight oil plays shows that Chinese ones are mostly in the Mesozoic and Cenozoic, characterized by strong heterogeneity of reservoirs and source rocks,also complex tectonic activities,but sweet spots are abundant, and reservoirs are diversiform.Moreover, basins near Cenozoic-Mesozoic orogenic belts are predicted as tight oil fairways.

AAPG Search and Discovery Article #90185 © AAPG Geoscience Technology Workshop, Revisiting Reservoir Quality Issues in Unconventional and Conventional Resources, Austin, Texas, November 12-13, 2013