The effect of oil emplacement on quartz cementation: evidence from Es3 sandstone reservoir, Dongying sag, Bohai Bay Basin, East China

Whether the presence of oil slow or stop quartz cementation is still debated. Microscopy, cathodoluminescence microscopy and fluid-inclusion microthermometry have been used on 11 sandstone samples(7 samples are from oil leg and 4 samples are from water leg) from Central Anticline Belt, Dongying sag(East China) to determine the influence of oil emplacement on quartz cementation.

Quartz overgrowth have been found both in sandstones from oil leg and water leg. Quantitative estimate on thin sections shows that the width of quartz overgrowth distributes from 6μm to 76μm in oil leg and 8μm to 56μm in water leg. Furthermore, in oil leg 40% of quartz overgrowth have a width range from 6μm to 20μm, 48% of them range from 20μm to 40μm and about 12% of quartz overgrowth have a width over 40μm. As a contrast, in water leg quartz overgrowth width range from 6μm to 20μm, 20μm to 40μm and over 40μm is 60%, 37% and 2% respectively. Moreover, quartz overgrowth volumes in oil leg range from 0.64%-0.90% with a average 0.73%, and in water leg the volume amount of quartz overgrowth are 0.18%-0.71% with a average 0.39%. Both oil and aqueous fluid inclusions were found trapped in quartz overgrowth or the detrital quartz/quartz overgrowth boundary. And the quartz cementation was precipitated at 95℃-140℃, accompanying or slightly postdating the main phase of oil filling. The variety of T_h from 95℃ to 140℃(approximate to the present-day formation temperature) indicating that quartz overgrowth don't be inhibited but continue precipitating to nowadays though oil has charged into sandstone reservoir. The positive correlation between the amount of feldspar dissolution pores and quartz overgrowth is because the oil fill with organic acids has promoted the feldspar dissolution which can provide silica for quartz overgrowth. Therefore, oil emplacement has no obviously inhibiting effect on quartz cementation, actuality, it provides source of silica for quartz overgrowth.