Lacustrine Shale Oil Enrichment and Mobility in the Lower Cretaceous Shahai Formation, Fuxin Basin (NE China): Integrated Inorganic–Organic Geochemical and Petrographic Evidence

TL;DR

This study explores shale oil potential in the Fuxin Basin, China, using geochemical and petrographic data to evaluate oil generation and mobility in lacustrine mudstones.

Contribution

The study provides new insights into shale oil enrichment and mobility in small-sized basins with oil-coal coexistence, using integrated geochemical and petrographic evidence.

Findings

Lacustrine mudstones in the K1sh4 member have high oil-generation potential due to Type II kerogen and midmaturity organic matter.

Oil mobility is restricted by strong oil adsorption onto organic matter, influenced by the biological origin of the organic matter.

Higher terrigenous input correlates with increased movable oil and enhanced reservoir connectivity in lacustrine shale systems.

Abstract

The exploration of lacustrine shale oil systems has become
a critical
frontier in global unconventional energy. Current research primarily
focuses on deepwater lacustrine shales in large-sized basins, with
limited attention to lacustrine shale oil in oil-coal coexisting strata
of small- to medium-sized basins. The Fuxin Basin, a small-sized sedimentary
basin in northeast China, is known for its cooccurrence of coal and
petroleum resources. The third (K1
sh
3) and fourth (K1
sh
4) members of its Lower Cretaceous Shahai Formation host archetypal
coal-measure and lacustrine source rocks, respectively. Therefore,
K1
sh
4 is an excellent research
object for studying this category of lacustrine shale oil resource.
To date, exploration of the Shahai Formation and related geological
studies in this basin have focused primarily on conventional clastic
reservoirs, coalbed methane, and shale gas, leaving its shale oil
potential largely overlooked. Focusing on 23 K1
sh
4 mudstone samples in Well LFD1, this study
integrated inorganic–organic geochemical and petrological data
to evaluate oil-generation potential, reconstruct paleoclimate and
paleodepositional environments, determine the biological origin of
organic matter (OM) in these samples, and reveal shale oil enrichment
and the controls on oil mobility in the K1
sh
4 lacustrine mudstones. The mudstones are characterized
by high OM abundance, mainly Type II kerogen dominated by oil-prone
sapropelinite, and midmaturity OM, collectively indicating substantial
oil-generation potential. Trace element and molecular geochemical
evidence indicates that deposition of these mudstones occurred in
a warm, semihumid to semiarid paleoclimate, within a stable, highly
restricted, predominantly freshwater (partly brackish) lacustrine
environment characterized by weakly oxidizing to weakly reducing,
dysoxic conditions. This depositional regime facilitated OM accumulation
and preservation, providing a material basis for shale oil enrichment.
High ΣnC21–/ΣnC22+ and low terrigenous/aquatic ratio (TAR)
indicate that OM is predominantly sourced from algal and prokaryotic
bacteria, while a low regular sterane/17α-hopane ratio underscores
the significant contribution of prokaryotic bacteria. Samples display
favorable shale oil enrichment (mainly medium-high oil content), yet
oil mobility is restricted by strong oil adsorption onto OM. The oil
saturation index is negatively correlated with ΣnC21–/ΣnC22+ but
positively correlated with TAR. These relationships demonstrate that
OM biological origin exerts an intrinsic control on oil mobility:
First, although algal/bacterial organisms serve as the primary biological
sources, a higher terrigenous contribution (relative to algal/bacterial
OM) lowers TOC and adsorbed-oil volumes, thereby increasing the movable
oil. Second, at equivalent thermal maturity, low ΣnC21–/ΣnC22+ and
high TAR values can be associated with larger pore sizes and enhanced
reservoir connectivity, facilitating fluid mobility and more complete
fractionation of n-alkanes within nano- to microscale
confinement. This ultimately leads to a more uniform distribution
of n-alkanes across different carbon numbers in terms
of content. This study confirms that the K1
sh
4 lacustrine mudstones in the Fuxin Basin constitute promising
targets for future shale oil exploration and provides a pivotal reference
for evaluating the lacustrine shale oil potential of the widely distributed
Shahai Formation.

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