Main Controls on Oil Accumulation in Tight Sand Reservoir: A Case Study from the Tight Sand Oil Reservoir, China
Commercial oil production in the Jurassic Formation tight
sand reservoir of Junggar Basin, China has been paid great attention by Chinese
geologists. The understanding of main controls on oil accumulation in the tight
sand oil reservoir is crucial to oil resource assessment. In this study, the
source of oil, reservoir evolution, oil filling history were investigated by
GC-MS oil compositional characterization, core thin-section lithological
observation, fluid inclusion analysis, carbon isotopic analysis of oils. The
core samples from three wells located in three different blocks were examined.
There are two main source rocks in the Junggar Basin,
China. One is the Permian lacustrine facies, which is characterized by Pr and
Ph ratio less than 1, 13C value of kerogen less than -30‰ ,
high Gammacerane index and high Beta-dammarane content. The other is the
Jurassic coal bed deposited in the paludal facies, which is characterized by Pr
and Ph ratio larger than 3, 13C value of kerogen greater than
-27‰, a trace amount of Gammacerane and Beta-dammarane. The crude oils
and extractives of sand from three wells Y1, C1 and D1 located in three
different blocks were collected and similar geochemical analysis were examined.
The biomarkers and isotopic values show that the producing oil of Y1 well is
mainly originated from the Permian source rock, in contrast, the producing oil
in C1 and the extractives of sand from D1 wells are originated from the
Jurassic source rock.
The fluid inclusion homogenization temperature and
buried history studies showed the Jurassic reservoir of Y1 oil producing well
was buried in shallow depth of 1800-3100m when oil was charged. The reservoir
retained about 14-18% high porosity according to the porosity evolution
historical curve based on basin modeling and diagenesis study. High porosity
provides available spaces for oil accumulation, as a result, high oil flow
acheives. In contrast, the Jurassic reservoir of C1 and D1 oil producing well
was buried at deep depth of 2300-4800m when oil was filled in. The
reservoir’s porosity is low and ranges from 7-14%, which is
disadvantageous to pool oil. As a result, non-commercial oil flow and even dry
hole wells achieve after drilling test in C1 and D1wells. Our results suggest
that commercial oil accumulation in tight sand reservoirs is up to reservoir
quality and abundant supply of oil from its source in the course of oil
charging.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California