We discuss a new class of polymer absorbents called "petrogels", polyolefin-based hydrophobic absorbents that demonstrate selective absorption of hydrocarbon (oil) molecules in water, counterpart to "hydrogels" that are known for absorbing aqueous solutions. To understand the effects of the molecular structure (i.e., polymer composition, cross-linking density, and morphology) to the absorption capacity with various hydrocarbons and mixtures, a systematical study was conducted involving polyolefin copolymers that exhibit a highly swellable network but no dissolution in hydrocarbons at ambient temperature. They include a set of semicrystalline poly(ethylene-co-1-octene) thermoplastics (metallocene-LLDPEs) with low crystallinity, melting temperature, and various morphologies (i.e., pellets, films, and foams) and a set of cross-linked amorphous poly(1-decene-co-divinylbenzene) (x-D-DVB) elastomers with low cross-linking densities. The absorption study involves pure hydrocarbons (toluene and heptane), refined oil products (gasoline and diesel), and Alaska North Slope (ANS) crude oil. In addition, the Flory-Rehner theory is also applied to determine the network structure (i.e., cross linking density and molecular weight between cross-links) and provide the structure property relationships. In the good solvent (absorbate) condition, the absorption capacity is basically controlled by the network structure. The maximum absorption capacities of semicrystalline LLDPE and amorphous x-D-DVB elastomer with toluene can reach 35 and 43 times that of the polymer weight, respectively. However, in the mixed hydrocarbon conditions, the absorption profile is determined by a combination of absorbate composition and absorbent network structure and morphology. With a specific LLDPE foam, for the first time we have been able to achieve the absorption capacity of ANS crude oil >26 times.